I have referenced the Carver Stereophile challenge - where a $600 Carver amp is nulled by Bob Carver (with RadioShack parts overnite in his hotel room) against a highly respected, SOTA tube amp
after adjusting frequency response and output impedance to match, Stereophile's "Golden Ears" couldn't tell the difference in blind listening
note - in their own listening room, with their own choice of source, speakers - and as experienced professionals taking money for making fine distinctions by listening the "test stress" argument doesn't work so well
so while the literal "all amps sound the same" is a strawman - amps can differ in frequency response and output damping by enough to be audibly distinguished under DBT conditions
but as understood and controllable differences it is "uninteresting" in an engineering sense - it means designers just build in EQ to give their amps unique "voices" - not that there is some mysterious superiority of audiophile tweaks, parts or tubes vs SS in amplifiers
"Carver Challenge" info/discussion:
diyAudio Forums - Blind Listening Tests & Amplifiers - Page 22
original Sterophile article:
http://www.stereophile.com/features/the_carver_challenge/