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
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