Having recently added a pro amp for two channel listening I will toss my opinion in. I think JohnK is 100% correct, up to a volume level. Having read those blind test, I can't tell if volume was ever made part of the test. I have found over the last few weeks (perhaps 20 hr of critical listening) that that level is much lower than I originally thought. Compared to my Denon 3300's internal amp (110 w/p FTC), the pro amp is capable of offering over 3 times the power (500+ w/c). While top end volume is clearly different [well, I have not found the "TOP"], there is more mid-bass even at the upper end of the normal (for me) volume range; before you get to really loud, 100 db constant.
To try to put this into context, I'm getting better mid-bass and bass at 93 - 97dbs than I was getting at the same volume from the Denon. [Heck, I'm even getting some driver over-extension at my normal volumes, which never happened before!] I think the dynamic peaks at this volume just jumped higher than the Denon could reach while driving the other frequencies, so it clipped them and thus reduced the power hunger bass notes. BUT - the newer Denon's seem to have stronger amp sections than my 3300, maybe this is not an issue now?
So I agree all well designed amps sound the same, but that extra power is more handy than I thought as dynamic peaks must be more demanding than I gave them credit for. Alan has a good article on the amounts of power needed to reach dynamic peaks, and that seems to be what I am experiencing. If you never see 95 on the meter, a new amp would likely go unnoticed as the amps will sound the same until you exceed the current amp's unclipped range.
