But the thing is, we're talking about amplifiers, not receivers.
A receiver, by definition, CAN and DOES alter the sound--with different processing, bass and treble knobs, etc.
I hate the reduction ad absurdum argument. Is a $100 receiver competently designed, with THD specs <1%?
Let's say this: run a Denon 3810 in pure direct mode and compare with a Denon 3810 in pure direct with an XPA-2 hooked up. Level match them. THEN tell me there's a day and night difference. (HINT: I really want to know this one).
Alternately, compare an XPA-2 with an A1400-2, again level matched, with no processing happening. Is there an audible difference, as long as neither is driven into clipping? I doubt it.