Hi Jon,

Sometimes an amp connected to a variac sounds
like crap.

I would second this, but would underline the word
"sometimes".

The most noticable improvement when going from variac
to normal mains was with a Klein+Hummel VS-55 amp
(6BM8 PP) I had to restore, which sounded like crap
at the variac, but got much better when directly
connected to mains.

Other amps (some with much higher power requirements
than the VS-55 mentioned above) showed no or only
hard-to-reproduce differences in sound w/ or w/o
the (same) variac.

I don't have an explanation, either.

Tom

--
A gleekzorp without a tornpee is like
a quap without a fertsneet (sort of).