I've listened to a lot of amps on a lot of different systems, and I would say that specs are probably less correlated with sound in the case of an amp than they are with virtually any other component on the system.
If someone put a gun to my head and said I had to pick a spec for an amp and make a purchase on that spec alone, I'd want to know its weight. That doesn't tell you a lot about it sound, but it tells you more than just about anything else you could name.
I just bought an amp here on Audiogon, and I couldn't tell you its power rating without googling it.