I have the dreaded hum

Try a different set of interconnects

Reconnect the old phonostage and see if the hum is gone.

I tried a diff set of interconnects, no change.
The dealer is sending me another pre-amp to try, but he still thinks something else is causing it.
Could it be that the extra phono stage is isolating a ground loop?

If you have a ground loop it will be in both channels. This is usually caused by poor AC ground design. Since one channel is OK, that's not likely it. Based on what I have seen so far in this thread, the problem is somewhere in the phono section itself. The swap of the phono interconnects (meaning: cables from the tone arm to phono input) rules out the table, and the problem moving from one channel to the other when you swap interconnect left for right puts it in the preamp.

Now there is a nuance worth checking: It sounds like you have a stand-alone phono section, if this is the case, what happens if you swap interconnects between the phono section and line stage? Does it move?

Yes => phono section *or* interconnect

No => line stage (input at least, unless it also hums on CD- you've not mentioned that so far...)

Atmasphere. When I use the stand alone phono stage the hum is gone. I only have hum when pre-amp is on phono input and switched to MC. That engages the internal step-up in the pre-amp. No hum with any other inputs.
All signs say it is the internal step-up, yet when I sent it back to the dealer he said he had no issues. The pre-amp was sent back to the manufacturer a few months ago for a different issue, came back repaired and apparently they checked out the phono section as well. At that time I did not yet have a TT so I didn't notice the hum. A few weeks later my brand new TT arrived and I have had a hum right from day one with the TT.
I will wait and see if I have any issues with the replacement Pre-amp the dealer is sending.