Why do amps sound different?

TIA - I am most certainly not any kind of expert, but I did sleep at a Holiday Inn Express (once...)

For starters, if they do sound different then they do not measure exactly the same - period. You may be referring to a single aggregate measure (amps, etc.) when you say that they're the same but if they sound different then the detailed measurements really are different. All amplifiers vary from the ideal of "a straight wire with gain" - it's in exactly how and where they vary that leads to perceived differences.

For one reasonably detailed explanation you could try this:
http://sound.westhost.com/amp-sound.htm

I would bet that they don't really measure the same on the test bench as you say, if they do the tests are not comprehensive enough. Specs can't be looked at individually. When people say that two amps measure the same, they often look only at WPC and THD and, for the very advanced, s/n ratio.

I am far from an expert, but from what I have read, measurements aren't always a barometer of sound. As a subscriber to Stereophile, many times the measurements indicated on some high priced amps are not really up to snuff, yet despite the measurements, the reviewer will swear on a stack of bibles that the amplifier sounded excellent. I think every company has it's own unique sound bias and will seek to design it in their product. The quality of parts and even the type of wiring (copper vs silver) will also determine the sound quality. For instance, even with the same measurments, tube amps almost always present a different sound quality than solid state.

We need to bring back Julian Hirsch from the grave to recite all his articles in High Fidelity magazine that covered that subject. Do you remember Julian? He was of the test bench school.