As the more astute readers may have worked out, the key difference between the two is the number of speakers. The PHI 21 is a 2.1 system, while the PHI 5.1 has the full complement of surround channels.
The satellites and centre channel each feature four midrange drivers with 50mm woofers, plus 'lightning fast', magnetically-shielded 19mm dome tweeters.
Bass frequencies meanwhile are taken care of by the subwoofer's 20cm driver, with the sub packing in some 100 watts of amplification.
Both systems come supplied with all the necessary shelf stands, floor stands, wall brackets and cables for instant – you'll be lucky, these things take time – installation.
The aluminium speakers come in a high gloss white finish, which the chaps at Infinity Systems reckon give the systems a 'timelessly retro 60s looks'. We shall see.
The PHI 21 will retail at £600 and the PHI 51 will cost £1000, and both systems should be in shops by the end of March.