matthewpiano:Firstly, the most important synergy in a system is that between the amp and speakers and I strongly urge you to pay most attention to this. IMO, the best path to successful system building is to start with the speakers and to then choose the electronics that (within your budget) give the speakers as much of what they need as possible.
The NAD amp won't be ideal for the Mezzos. They are very revealing speakers. The 740A is well worth the extra. Its a much better amp than the NADs with more clarity and a real sense of ease whatever you play through it. It never sounds strained or out of control and the 100wpc output and large toroidal transformer give it plenty of dynamic headroom.
The NADs are superb budget amps, but the Cambridge is the most affordable way into high-end sound and works brilliantly with the Mezzo range.
NAD amp + Mezzos is not a very good match IMO. The Mezzos are very revealing speakers and their low distortion, accurate sound is much better served by an amp with similar characteristics. The NAD amps are more coloured in sound and work better with B&W 6 series speakers.
The Rega Brio 3 is an excellent little amp but its tendency towards becoming strident when the music gets complicated puts it well behind the 740A.
The Audiolab is also very good and is probably the best of the bunch after the 740A but it doesn't quite have the airiness and sense of ease that the Cambridge does.
Whichever way you go, some auditioning is vital. Let us know how you get on.
I normally agre with most of your comments Matthew, but I have to disagree with this! A different presentation than CA yes of course, I might argue to my ears that CA stuff is way too light and weedy, but that does not make it "coloured" (and therefore by inference inaccurate)!.
Ultimately, we are making subective comments about a very subjective issue (music), so the best way as ever is toi listen oneself, maybe using coments and reviews from others as a guide only.