I have the dreaded hum

I agree; that circuit is usually independent and thus is isolated as the problem. Don't let those reps jerk you around and good luck.

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...)