Somewhere on this forum a few months ago I was watching a video on the building of a McIntosh tube amp. Before the transformers were installed they were turned upside down and a black tar like goop was poured into the bottom of the bells which covered any plates or windings. If I'm correct it was to stop mechanical vibration within the transformer.
I didn't even notice the output transformers in my Dynaco. If all totaled up there are 10 missing transformer lag bolts and lockwashers, rubber washers, washers,and nuts securing the outer bells. The Mark 3 transformers I just looked at are the same way, each one missing 4 bolts.
As a project I am going to try to find those bolts, install them, and see if some hum goes away. It may be far from your problem but an easy project for me to try troubleshooting.
Why would David Hafler, the founder of Dynaco who's forte was the Ultralinear transformer, skimp on a few bolts holding the bells secured on his trannys?.....take care and good luck fixing the hum what ever the problem is......Mark Korda.