Do you think some amps are warmer than others, or more forward than others? In my limited experience with comparative amps, some are more warmer and some have a "more in your face" attitude than others. Am I off base here?
The basic theory is that two amps will sound alike unless:
1) One is playing louder than the other.
2) There are audible differences in frequency response (which can often be caused by impedance mismatches, particularly in the case of tube amps).
3) One is clipping.
So if you hear an amp as more forward or warmer (whatever that means to you), it has to be caused by one of the conditions above.
Now, here's the thing: Among modern solid-state amps driving typical home speakers, it is very rare for conditions 2 and 3 to hold. As for #1, that has nothing to do with the character of the amps, and everything to do with where you set the volume control. Tweak it a little, and both amps will sound the same.
So your next question is, OK, if that's all true, why do I and other audiophiles hear differences between amps? Three reasons:
1) You're comparing without matching output levels, which has to be done very precisely (i.e., your ears and/or a Radio Shack SPL meter aren't enough; you need a voltmeter measuring the signal at the speaker terminals).
2) You aren't comparing the two side by side, but are relying on your long-term memory of the sound of one of them. Our long-term memory of subtle sonic differences is really poor.
3) You are subject to what the psychologists call "bias," which simply means that your hearing perception is influenced by other factors--specifically, other things you think or know about the amps. If some salesman once told you that Brand X tends to be warm, that's liable to affect how you hear Brand X. Can't be helped, as you're only human. That's why scientific listening comparisons are always done blind.
Needless to say, all of this is highly controversial within the audiophile community. And since it's only a hobby, it's perfectly OK to ignore it. But nothing I've said above is even remotely controversial within the scientific community.
Now, pardon me while I don my flame-proof suit.