this is an issue that has been strongly debated on every forum I've ever read at one time or another.
To answer your question you have to remove any processing the two amps may have from the equation -- high pass/low pass filters, bass boost, eq's etc...
If you take that into consideration - what "proof" (scientifically controlled listening tests) there is says as long as MEASURED power is the same there should be no audible distinction between two amps of similar topology.
There are those, however, who still claim to be able to hear these differences - which is where the "strong" debates usually get started.
The price tag of an amp can't really be attributed to its SQ or lack thereof, but more to overall quality, more than advertised power, precision processing capability, durability, good technical support etc.
ideally the amplifier should be completely transparent, meaning the amplification of the source signal should not color the final output at all. both amps with the same power *should* sound identical.
this isn't the case though, as cheaply made amplifiers do often color the output, affecting the sound versus the original source material. The better the amp, the less it will alter the sound. Some companies even offer EQing on the amps to purposely color the output.