FH, I didn't want to extend this thread still further, but a couple of comments are in order. Certainly a much higher noise level in one receiver could in some circumstances be audible in quieter passages, but manufacturers specify noise and distortion ratings(and a few reviewers actually test for them)which are generally inaudibly low and in any case don't affect the other aspects of sound quality.
What you termed "attack and decay" would in fact show up in frequency response graphs if it was inadequate in some poorly-designed amp. In order for an amp to reproduce a frequency at a flat level and with low distortion, it has to have a high-enough "slew rate", which measures the ability to rapidly supply changing voltage requirements at different frequencies and levels of loudness. If it can do 20-20,000Hz flat and with inaudibly low distortion, that establishes that it has a high enough slew rate. No differences in "attack" exist in such items.
