Peculiar buzzing in 410-65

I recently bought a near-mint 410-65 on eBay. (Chassis 2275-65) The thing looked like it just came out of the showroom -- no rust, no scrapes, no scratches, even knobs look brand new.
I took it to my amp tech; he re-tubed, re-biased and replaced caps as necessary.
But there's a problem: When I strike a note from low "A" down, the note goes fuzzy (like a cheap fuzzbox) and sustains for a very long time. We've checked the speakers; they're fine. We plugged the chassis into other cabs; the buzz occurs in all other cabs.
Tech suspects output tranny. We have another 2275-65 chassis we can rob for a tranny.
Before he starts swapping trannies, do any of the learned techs here have any other educated guesses?
Thanks,
WJ