Why do amps sound different?

Although alluded to before, it seems that the amount of negative feedback is a big issue.

2 amplifiers can easily have the same bandwidth, but one running with feedback and one without. The one without will likely sound more relaxed, since it lacks the global feedback which enhances odd-ordered harmonics which in turn behave as loudness cues. We are not talking a lot, like Kurt says- hundredths of a percent is all it takes to make the difference.

A further complication is the idea of 'constant voltage' output, which is the same as doubling the output power as the load impedance is cut in half. Some speakers are designed to expect this (B&W 802). Other amplifiers are designed with the idea of "constant power" in mind- that is that power does not change regardless of the load (tube amps are good examples of this). Such amplifiers, sometimes referred to as 'current source' amplifiers, have a higher output impedance and an entirely different class of speakers exist to accommodate them (Sound Labs and horns for example).

In fact two paradigms of design and measurement exist in audio today:

http://www.atma-sphere.com/papers/paradigm_paper2.html

This is about more than just simply matching components, but that is what you have to do. Normally one paradigm will take over in a field of endeavor but that did not happen in audio because the 'prior art' (tubes) did not go away like they were supposed to- too many people like them.

Another interesting thread with great contributions by everyone.

Is someone able to explain the difference between "current paradigm" and "voltage paradigm" amplifiers?

Amfibius, take a look:

http://www.atma-sphere.com/papers/paradigm_paper2.html

'Current source' is a Voltage paradigm term for amplifiers that have a high enough output impedance such that they exhibit (or at least try to exhibit), constant power with respect to load. Usually this refers to a tube amplifier, but not universally.

"Voltage source' is again a Voltage paradigm term for an amplifier that can express a constant voltage regardless of load. Another way to put this is that power is doubled as the load impedance is halved (or halved as the load impedance doubles). Usually this refers to a transistor amplifier, but again, not universally- there are always exceptions because this is not about tube/solid state.

I never look to specs other than output power to make an amplifier purchase since specs are seldom an indicator how an amp will sound. Most quality amps are designed by ear, the mfgr applying different circuit topologies, trying different electronics parts & wiring in critical areas of the circuit to reach a desired sound before production begins. You end up with the designer's opinion of what sounds good.

Phd, one of the distinctions of the Power paradigm vs the Voltage paradigm is under the Voltage paradigm, the specs often have no meaning as you already know. Under the Power paradigm, the specs correlate with what you hear.