Well, the upgraded transformer is a failure. Turned on the system this morning and heard a slight hum. Cranked up the volume, and yes, it's definitely there.
I have a early 70's class A bipolar amp upstairs that has always been very susceptible to hum issues in anything connected to it.
Brought the preamp upstairs and connected it to the bipolar. Yup, really pronounced hum. Went through every troubleshooting procedure I could think of, no joy.
Pulled the large transformer, reinstalled the origonal. No more hum with the units seperated, very slight hum with the units stacked. Typical of the approximately 50 years I've had the amp. The amp has two big transformers and induces slight hum in anything stacked on it.
I really liked the way the preamp performed with the big transformer, but won't tolerate hum in the main system. Guess I'll look at mounting it off board. Really don't want to, but that's the only way it will work. Need to research enclosures and connectors.
No big deal in the large scheme of things. Learned a lot trying to make it work.