I can confirm that removing the back tube and leaving the unit on for 48 hours (no signal, just turned on) will make the 6SN7 (front tube) go totally silent. Kind of a pain but the way to go.
This amp is an outstanding bargain so it's well worth taking the time.
Silence is golden as they say.
Thanks. I tried it overnight that way and the input tube was silent in the morning--but not the next time I tried that tube! I'll give it longer burn in when I have time.