Weird hum 410-65

I posted this in another area, but didn't get an answer. Thought I'd try here. Any and all help appreciated.
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

I suspect a bad solder joint somewhere personally. I would run a low hz signal through the front end and start tracking the signal through. You'll obviously need a generator that goes down far enough that it simulates the "A" note and below which I would think is in the range of MOST generators. Does it do it at ALL volumes? Not just loud? Does this have a 12ax7 PI tube? If not, I would look at the driver transistors then. These are a matter of tracking the signal through. It COULD be a transformer but that problem is not what I've seen when OT's go. He can measure the resistance of one side of the OT and compare it to the other as a rough chack and of course if it's shorted to the spool(ground). Come to think of it, that last time I saw one of the tupe PI amps do it, I tracked it down to one of the IC's taking a crap. Just a thought. Mike.

Same thing happened to my 2/12/65 (with 12ax7) some years ago. My Tech with through my amp and found a lose solder joint and, that was that. Hope this helps, Good Luck.
Dave

Thanks Mike and Dave. The chassis is on the bench, so I'll mention a re-check for loose solder joints.
I know he's run signal through it and scoped it, but he's found nothing unusual.
The buzz is obvious at any reasonable playing volume, not just loud.
Amp has the 12AX7.
Surely could be an IC. Tech probably knows, but I don't: How does one find the guilty chip? (I've got another 65 chassis to rob parts from.)
WJ

You may have to "ping" the amp (hit it with a tone burst then a period of silence so you can see the problem on the signal decay). Running a continuous tone from a signal generator may not excite the problem into occurring.
This could be a bad solder joint or perhaps a bad coupling capacitor. If an interstage coupling cap is leaky (checked with a voltmeter, output leg to ground) this can be found fairly easily.
Steve

For those who are interested: It turned out that my amp tech was right with his first guess. The output transormer was shot.
Tech put in a new Mercury Magnetics MM clone transformer ($150 and well worth it). The amp sounds like a million bucks, much better than my other two vintage MMs, and better than I ever remembering my old MMs sounding back in the 70s - 80s.
WJ