I have the dreaded hum

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...)

So the phono preamp is a stand-alone and is OK.

I'm just trying to be clear here.

And the 'preamp' is full function and has a LOMC input? If nothing is connected to the inputs of that preamp, does it still hum when its in the MC position? Its OK in MM position? How about CD?

Well, you've already tried other phono/arm cables. And they work with your outboard. So the problem is in the preamp, whether the dealer thinks so or not.