Sources: iPhone4 alone, iPhone4 with v-moda Verza DAC/amp, various computers using the HRT Microstreamer DAC/amp and the Microstreamer's headphone out.
First impression of the Sol Republic Master Tracks ('SRMT' from here on): Bass! The kind you don't have to argue about. It's there in quantity for any need you might have. That aside, I see this SRMT as 2 headphones in one (a bargain BTW) - the extra-bass model for gaming, TV action film, house and other bass-centric music, and the hi-fi model (using bass reduction) for symphonies, folk and acoustic, jazz, rock/pop/metal, and other such delicate genres. Unless otherwise noted, all comments below apply to the SRMT using bass reduction, since I listen to music only, and my tastes are mostly towards neutral reproduction.
The SRMT sounds to me like a blend of two other headphones I have - the Marshall Monitor and the Beats Mixr. The SRMT's bass is extra strong like the Mixr, and when bass reduction is used, still has a stronger bass (including the deep bass) than most so-called high fidelity headphones. The rest of the range above the upper bass sounds more like the Marshall - darker than the Mixr, especially in the "presence" region or lower treble. Most other reviews I've read of the SRMT don't describe the sound very precisely, comparing to other known references like the Mixr, or even the old school headphones like the Sennheiser and Beyer models. I have a feeling that any critical reviews of the SRMT by the best-known establishment reviewers will find the combination of extra strong bass and some shyness in the lower treble - that will take points off of their ratings for this headphone.
In my case, using bass reduction for critical listening at home, the balance is perfect, or as near perfect as anything else I have, and that's excellent. The sound overall has a darker tonality than the average headphone, including the Beats Mixr, and it's about the same as the Marshall Monitor. The reasons I believe the SRMT is a 2-headphones-in-1 bargain is because 1) The SRMT's sound using bass reduction is excellent, with no bothersome peaks or dips anywhere in the music frequency spectrum; 2) The sound played flat (no EQ) provides the extra bass that gets lost in many portable use situations - outdoors or on public transport for example; and 3) Where most "bassy" headphones don't have the extra strength in the deep bass that they have in the upper bass, and using bass reduction with those headphones results in a weak lower bass, the SRMT's lower bass remains solid with good impact. The only other headphones I recall having this good of a bass response are the v-moda M100 and the Beats Mixr.
The overall sound of the SRMT is very smooth from top to bottom. Unlike some reviewers who go into minute detail about the many aspects of sound the customer is likely to experience with their new headphone, I stick to the things I can explain - the things that anyone can hear on their music player, computer, or portable headphone amp etc. Besides having a bass response that's solid and detailed (given the things I noted above), and a midrange that's just right (neither forward nor recessed), the SRMT's treble is also ideal - just strong enough with enough detail for reproducing the fine upper harmonic detail in voices and instruments, but not so strong as to make sibilants or other treble irritations from lower-quality music tracks bothersome. I rarely say anything about amps except that they improve the sense of space or "air", but with the SRMT and my USB DAC/amps, the highs dried up more than when playing with the iPod alone, and I got better results with the FiiO E07k through the 'Aux' port.
The SRMT's soundstage seems at least average, which is good for a closed-back headphone. Isolation also seems average for a closed headphone, and the leakage is low - low enough that playing the SRMT at a moderate volume level in a cubicle next to other cubicles in a quiet office should not be a problem. The music track examples listed below will tell my impressions of the SRMT's sound with that particular music. If you should happen to read other reviews on the SRMT that describe either the mids or treble as recessed, consider that those reviewers are playing the SRMT with the bass full up and that I'm using bass reduction. The reason I'm not reviewing the sound of the SRMT played flat (no EQ) is because many other reviewers have already done that, and I want to provide an alternative for customers who'd like to have a headphone like the SRMT, but wouldn't like extra-strong bass.
The SRMT comes in several colors, and my video review shows the plain black version I purchased. The SRMT seems to be mostly plastic, including the headband I think, but the headbands are user-replaceable and are easily found at retail stores. I consider the build quality to be as good as the sound - excellent. The earcups are circumaural (around-ear), and they fit around my average or larger size ears with great comfort. There are 2 main classes of circumaural earcups in my experience - the large round cups like the Beyerdynamic DT770 and other Beyer series, or the Soundmagic HP100 and HP200, to name some examples. Then there are the smaller oval earcups/earpads like the Sennheiser Momentum, the Phiaton MS400, the v-moda M100, and the SRMT. The SRMT fits me better with greater comfort than any of these other small oval earcup/earpad designs.
The headband clamp is fairly light for this type of headphone, and it's also very secure on the head, but the snug and secure fit may have a price - in very warm weather the earcups may feel overly warm. The 4-ft. straight cable is detachable and double-side entry, and the 2.5mm plug that goes into each earcup looks like a stereo plug, although the extra connector may have something to do with the start/stop and volume controls. The other end for the computer or music player is an Apple-style right-angle miniplug. The SRMT doesn't come with any sort of carry case, just a thin plastic-coated bag that won't protect the headphone from anything. If you need to take this headphone with you to various places and you don't want to wear it around your neck (that works very well BTW), I'd advise taking it apart - earcups and headband and cable - and packing them into whatever protective container you can find.
In other reviews I've done I've included the following music examples with comments about how the headphones sound with each track. My suggestion is instead of reading each one as an absolute unto itself, you could compare my notes here to other reviews and see how the SRMT compares with each individual track. I copied these examples from my Beats Mixr review and then went through each one in a listening session, checking for any differences I heard from what I reported for the Mixr, and found only one or two minor differences - an excellent result. The SRMT is now one of my 4 favorite headphones, which includes the Beats Mixr, the Marshall Monitor, and the v-moda M100.