Youtube review: http://youtu.be/Ckn0e6jnMTM
Sources: iPhone5 alone, iPhone5 with FiiO E07k using LOD, various computers using the Microstreamer and Audioengine D3 DACs/amps.
Review note: My initial impressions of the sound of the T51p are based on direct comparisons to other headphones - in particular the Shure 1540, B&O H6, B&W P7, Sennheiser HD380 Pro, KRK 8400, v-moda M80, Thinksound On1, and notes I've accumulated from many prior reviews. I describe how I relate to the T51p (i.e. my personal tastes and how I use the headphone) only after covering all of the objective issues.
First impressions of the Beyerdynamic ('Beyer') T51p: Very dark, very recessed treble, and boomy bass. Played flat (no EQ) I don't think anyone would be able to hear the upper harmonics that give voices and musical instruments their characteristic sounds, with their representative attack and decay envelopes. So while I play the B&O H6, B&W P7, KRK 8400, v-moda M80, and many other headphones without EQ, the T51p requires a treble boost just to qualify for minimal hi-fi listening. Even then the lower treble (i.e. 'presence' area) is recessed to the point that many of my music tracks sound unusually distant or nearly lifeless. Compounding the issue with the lower treble is a ~10 db peak around 5 khz, and when music has sufficient energy around 5 khz, the volume has to be turned down making the adjacent frequencies much less audible. That's the bad news. There's some good news though.
I had two of the Beyer DT1350 headphones, and I replaced those with other models because the earpads were somewhat stiff and didn't make a good on-ear seal for best bass response, particularly in cooler weather outdoors. Since then Beyer has introduced a new version with better earpads, and this T51p being nearly identical physically to the DT1350 inherits those same improved earpads. These new earpads are comfortable and seal very well - in fact there is essentially zero leakage of sound with the T51p even at loud playback volumes. So with a simple treble boost on iPods and iPhones, and playing music in a quiet place with no distractions, I find the T51p's dark sound enjoyable and sufficiently detailed to provide a reasonable degree of fidelity for most recordings. Doing some experiments with computers using Foobar2000's equalizer, I found that the following EQ settings improved on the i-device EQ (which couldn't suppress the ~5 khz peak:
+2 db at 2.5 khz, +4 at 3.5, +2 at 5, +4 at 7, and +6 db at 10 khz and above. The upper treble response doesn't fall off steeply above 10 khz, so I don't see a need to increment the sliders there.
My final judgement of the T51p's sound (for the forseeable future) is: Very good on computers with appropriate EQ, since the T51p's Tesla technology provides a lot of headroom for adjusting the frequency spectrum differently at different points. I don't recommend adjustments much more complex than what I described above, since complex adjustments lead to large narrow peaks and recesses at frequencies adjacent to the sliders' center frequencies. Some experts recommend 'parametric' equalizers for this kind of task, but without expensive test gear to perform automatic measurements, it wouldn't be practical since it requires a lot of time and effort. My judgement of the sound when used with i-devices and other similar portable music players is: Very good for outdoor use and when riding on public transport, since the headphone's emphasized bass compensates for the loss of audible lows in environments that have strong ambient low frequencies.
Since soundstage and certain other sonic properties are dependent to a large extent on a reasonably full treble, I can't say that the T51p's soundstage is anything but average under best listening conditions as described above. But then again, for nearly any good quality headphone, the soundstage will vary widely and mostly according to the recording rather than the headphone itself. The exceptions are certain designs (i.e. Sennheiser HD800) which exaggerate the soundstage using extra-large earcups and highly angled drivers, so that the sound is bounced around some before entering the ears, like speakers in a room.
In my worst-case listening situation, using an iPod or iPhone only, the bass is somewhat boomy and the midrange slightly veiled. Listening with a decent amp attached to the i-device Line Out Dock (bypassing the i-device volume control and headphone jack), the sound is greatly improved with better soundstage and frequency extension, especially on the treble end. Stepping up from there to using a computer and a USB mini-DAC like the Microstreamer or Audioengine d3, the boomy bass is better controlled and the highs become more detailed and extended. Isolation with the T51p is very good - more than 10 db at midrange frequencies and a lot more higher up. Leakage is so low that it's essentially nonexistent.
The T51p looks very similar to the DT1350, but where the DT1350 split headband actually splits, the T51p's headband does not. The DT1350 cable is single-sided and partly coiled, while the T51p's cable is dual-sided and straight, about 1.2 meters long. Construction of the T51p is basically all metal and very high in quality, but the cable is thin and if tugged hard enough times, would likely fail. I don't see any suggestion that the cable is user-replaceable as are the earpads and headband pad. The earpads (new Beyer design) are soft and squishy, and slightly wider than the earcups and the older earpads, so a perfect fit is easy to attain and comfort assured, along with high isolation and low leakage. The carry case that's supplied with the T51p is a velcro-close stiff case that completely protects the headphone with no openings, is very compact and flat, and will fit easily into most airline carry-on bags or student backpacks without taking up much space in those bags.
The music tracks below were listed in several prior reviews, and are a random sample selected from the 400 most recent tracks I've acquired. Since these tracks cover a wide range of genres and were selected when I was using several different headphones, there won't be a bias toward the T51p headphone with this music. My suggestion is instead of reading each comment below as an absolute unto itself, you could compare these notes to those prior reviews and other reviews as they get posted, and see how the T51p compares with each individual track.
Ana Victoria - Roxanne (Pop Vocal): Spacious sound, good bass tone and impact, and the vocal sounds natural. Good reproduction by the T51p, although the 'presence' region around 3 to 6 khz is recessed.
Ben Goldberg - Root and Branch (Jazz): Realistic you-are-there sound with great instrumental reproduction. The T51p plays this very well.
Benedictines Of Mary - O Come Emmanuel (Medieval/Female Choral/Acapella): Very spacious sound and natural reverb for a large recording venue (cathedral). The T51p presents the voices realistically.
Black Sabbath - Iron Man (Classic Rock): Excellent instrumental detail - the vocal sounds very natural. As with most classic rock tracks, there is very little or no deep bass. The T51p plays this music very smoothly, and the lack of deep bass doesn't unbalance the treble.
Candy Dulfer - Lily Was Here (Jazz): Narrow soundstage, but excellent detailed instrumental tone. The T51p gives this a reasonable sense of space, but in spite of being a modern recording, the net effect is only slightly better than enhanced mono.
Cantus - Danny Boy (Traditional/Male Choral/Acapella): The T51p plays the voices with enough low end warmth and weight to sound very natural. In spite of my impression that the T51p has a strong bass, there is no exaggeration of the low end of the male voices on this track.
Chris Isaak - Wicked Game (Pop/Rock): The T51p plays this high treble energy recording with subdued brightness - the voice and instruments are detailed but not sharp or edgy.
Daft Punk - Lose Yourself to Dance (Electronic/Disco): Less than hi-fi quality recording, but the voices are very good. There's a good amount of bass impact, but the bass doesn't have much detail.
David Hazeltine - Fur Elise (Jazz): A very high-quality recording from HDTracks. The T51p reproduces the instruments smoothly with a gentle ambiance. The wire-brush-on-cymbal harmonics are very extended but soft.
David Lynch-Lykke Li - I'm Waiting Here (Soundtrack/Vocal): Dark, moody song - Lykke's voice is very detailed, the strong bass impacts are very good, but most of the instrumentation is soft and kept in the background. The T51p plays this music extremely well given the sonic limitations.
Dream Theater - Take The Time (Metal): The sound quality here is limited, but the T51p is smooth enough to bring out the details in this very busy music without verging on harshness.
Genesis - Follow You Follow Me (Pop/Rock):The T51p plays this old and less-than-ideal recording well enough to enjoy, but the soundstage is fairly narrow.
Giant Drag - Wicked Game (Pop/Rock): Annie Hardy's version of the Chris Isaak hit has a lot of energy, but the quality is limited - still the T51p pulls out enough detail to be a pleasant listen.
Grieg (Beecham-Royal Philharmonic) - Peer Gynt-Solveig's Lullaby (Classical): This very old (late 1950's) stereo recording must have been made on the most expensive gear in the world, since the overall sound quality and especially Ilse Hollweg's amazing voice are as close to "being there" as I've heard with some of the better classical recordings made since the year 2000. The T51p plays this softly but pleasantly.
Hubert Kah - The Picture (New Wave): This track has great bass detail and weight at the same time, which I find unusual for this type of 1980's pop music. The T51p plays this music very well.
Hugo Audiophile - 15-16 (Electronic): I'm not sure what the 15-16 stands for - perhaps track numbers from a CD album. The strong deep-bass tones that start around 33-34 seconds into the track reproduce fairly well with the T51p. This is a great recording for evaluating whether a headphone's bass will be sufficient for most environments, since for many headphones that have a weaker bass, the deep bass gets absorbed and mostly lost when the environment contains a lot of low-frequency energy.
Korn - Another Brick In the Wall (Rock): Aggressive rock that's very satisfying for hard-rock fans. The T51p plays this near perfectly, which is to say, with proper edginess and bass impact, yet without unintended sonic harshness.
Kunika Kato - Fur Alina (Vibraphone): A very unusual instrumental - the tone quality is unlike anything I've heard before. Recording close-up is part of the magic here, but the T51p does the rest in reproducing the full harmonics of this amazing instrument.
Michael Buble - Nice 'n Easy (Easy Listening/Jazz): This is the only track I bought by Michael Buble, but it's a great recording and vocal performance. The sound of the backing band here is rendered extremely well by the T51p, and the voice isn't pumped up for Loudness Wars thankfully.
Michael Tilson Thomas - Rhapsody In Blue (20th Century Classic): Great sound and soundstage, and terrific piano playing and tone. There are some very deep bass impacts starting around 38 seconds into the 17:24 length track, and the weight of those impacts is just discernable with the T51p.
Muse - Madness (Rock): The bass in this track has good impact and detail with the T51p, and although the voice is somewhat forward, it doesn't interfere with my appreciation of the bass line here.
Phaeleh - Afterglow (feat. Soundmouse) (Electronic/Vocal): The instrumental sounds that begin this track are played very nicely by the T51p, but the voice tends to overwhelm those background sounds - until the heavy bass impacts kick in. If there is any doubt about whether the T51p will play heavy impactful bass with good detail (if such sounds are really in the recording), this track is the proof. If you were to begin your T51p listening with this track, you might think you were listening to a headphone that has a very boosted but tight and detailed bass. Simply amazing.
Richard Strauss (Mester-Pasadena) - Also Sprach Zarathustra (opening) (Classical): The granddaddy of bass is in the opening 1:50 of this recording, and I've heard it only once on a large and expensive loudspeaker system in Cleveland. For most people, that experience would be indistinguishable from being in a fairly strong earthquake. The T51p conveys as much of that experience as is possible on an average headphone. The tympani also have good impact here.
Sargis Aslamazian - The Sky is Cloudy (Classical/Armenian): The National Chamber Orchestra of Armenia has a great classical program, and the T51p plays this music with good separation, tone, and big-orchestra precision.
Satri-Tomoko Sonoda - All The Things You Are (Jazz): This track came from Bakoon Products, who make high-quality audio amplifiers. There's a lot of upright bass plucking in this track and the T51p plays it fairly well, although it's recorded pretty close-up and sounds boomy at times.
Tommy Smith - Johnny Come Lately (Jazz): Small-combo jazz - sax, piano and drums. The sound is fairly close-up but well-recorded, and sounds very nice with the T51p, although the wire-brush-on-cymbal harmonics are not as extended as on the David Hazeltine track above.