Relationship of amplifier sound to transformer quality?

As above.
Paralleling power supplies is a problem waiting to happen.

a] a quality output transformer is the heart of any good tube amplifier.b] do not try to combine the outputs of two separate transformers (AC power nor audio output). they will both be unhappy to say the least.c] do not try to combine the outputs of two separate AC or DC supplies. They will be more than unhappy.