emailed GrooveTubes tech support and received this reply:
"Thanks for your inquiry. You should contact a qualified tech before you replace 6V6 tubes with 6L6 tubes. You could damage your power transformer. The amp may need some modifications for it to work properly. You would definitely need to get the bias set for the new tubes.
Hope this helps,
And finally, forwarded the GT email to my local music store, who replied:
"Although a 6V6 and 6L6 will replace each other and work, the amp would need to be re-biased and in the case of the Excelsior it is a fixed bias and cannot be adjusted. I suppose it is something they did with that one to keep that vintage, pawnshop sort of vibe. So with some amps no problem but shouldn’t be done with this one."
The manager offered to swap back the 6l6s I had bought for a set of 6v6s - and threw in a set of strings to make up for the confusion.
So it's been another educational trip for me. I appreciate all the feedback - I should note there is a lot of valuable info on the GT website.
Isn't that what we all told you?