Those are great amps!
Chuck, were the two transformers coplanar? Moving it to the bottom of the cabinet solved the problem, so it's all academic, but I wonder whether rotating it 90 degrees might have helped, too?
If you look closely at the chassis, you can see the transformer mounting holes for the power and output transformers, note that they are at 90˚
Remounting one or the other was the only way to completely eliminate the hum.
I've heard that this (meaning this circuit, but in the "Supro"
chassis) was the amp Jimmy Page preferred on his Telecaster, (though the
internetz are now littered with conflicting information about that).