"I did not 'expect' the amps to sound different. I did not 'expect'
that any given amp would have any particular sound. That amps sound
different was the conclusion I drew from this experience.
"

Were you of the view before starting that all amps sound alike, or was it
that you assumed some amps do sound different? If you had been reading
the hi fi rags where amps sounding different is standard fare, usually
based on a single anecdotal testimony of an article writer, then you need
not make any decision on purpose beforehand. The very fact you have and
use a vocabulary of amp "sounds" suggests you have had this hi fi rag
experience, even if you had decided to have no opinion beforehand. Doing
a dbt excludes this or any other possible expectation bias, if ithere is
no such bias, doing a test should get the same results as you report.

"By the way, I asked a friend to listen along with me. His opinions were
exactly the same."

You are aware no doubt, that if the friend didn't listen from the kitchen
the results are not valid, grin? This even makes the case more
problematical, mutual reinforcement in such task situations only serve to
multiply bias effects.