Why do amps sound different?

Generally if there is an audible difference then a difference can be measured. Specifications are however measured under conditions that are not typical of when a power amplifier is used to drive a complex load (speaker).....hence the possibility for different sound. In general, good pre-amps should sound exactly the same - if they sound different then one may be suspect or there is a mismatch between rated input and output levels and/or impedance with other connected components. Often a distinguishing feature between a good preamp and an excellent one is better channel separation, immunity to noise (from dirty power) and lower noise floor.

Note that the other way round is not true....often a difference can be measured that is inaudible when listening to music.

Cost. Every amplifier is designed to a price point. Since there's no such thing as a perfect amplifier, each manufacturer strives for the best sound within a budget (assuming that's the design criteria). Everything inside the amplifier imparts distortion - there's no getting around it - so they compromise as best they can. Forget bench numbers (see: Tube Amps), the sound of an amplifier is mostly based on what the designer hears as he's voicing the amp. And since no two people will neccesarily concur what is the best way of doing things, there's will always be differences - whether it's amplifiers or pizza.

I'm thoroughy convinced that we don't know how to measure some things that really count- and that's why amps sound different at similar specs.

What are those "things"? I have no idea.

My bet is that the perception of reproduced music is extrememly complex, and we humans are still not as smart as we believe ourselves to be.

Amps are like ice cream flavors. I agree with most threads here but, amps embellish certain freq. ranges to tickle our ears. We like what we like! Most really nuetral amps don't sell that well.

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.

Nah, leave him there. It's threads like this one that demonstrate how few people understand what the test bench tells us and doesn't tell us.