Soul, i'd have to back alot of what John has mentioned here. In the various tests i had in the past of a few amps and receivers, using an instantaneous A/B switch, and when adjusted for sound pressure levels between amp units, there was a negligible difference in audio quality. That is to say, no one could tell them apart with enough accuracy to conclude a statistical probability of an audible difference.
I know that some audio buffs won't believe my word, but having done such a test myself, the proof is very clear.
John does comment about sound levels and amp power but with an assumption of a relatively close listening distance and that SPL of say 100dB is too loud. I sometimes listen to music while working in the kitchen 18 feet away and even when i'm sitting closer, i like 90+ dB SPL. As such, power IS a requirement. I've had a few runs with some lower power amps as well, 50 -80 wpc range and frankly, while sitting at my home office desk listening to music at SPL of a comfortable 75dB these receivers started to clip.
Not all amps/ receivers are made the same when it comes to power reserves, but i do believe they all sound the same when played within specs as John commented.
Now speakers, that is a whole different ballgame. There can be quite a huge difference between speakers and sound (and similar to wine, cost does not correlate with quality except in one's own subjective mind and what defines quality...around these forums it usually relates to linear response as the goal for ultimate speakers). Surprisingly, the difference between speakers is often not as 'night and day' as is often described. I think many of the Axiom folk who attended the last reunion that had a chance to hear a blind test of Axiom speakers vs. some B&W, realized just how small those differences are.
Room is a huge factor. Heck, position IN a room is a huge factor. I would definitely look to speaker position and room acoustic treatments (moving a couch, carpeting/floor rugs, wall curtains or specific sound treatments) as the first ways to look at changing the sound response (aside from considering other speakers).
