Need help diagnosing a preamp problem

Check your turntable.

Switch the Right/Left of your phono cable to the preamp. If the problem moves to the left channel then the problem lies with the turntable. The tonearm cable might be damaged. Possibly a cartridge issue.

Also, did you make any recent changes? Or disturb any cables/connections prior to the Right channel going out?

"Also, did you make any recent changes? Or disturb any cables/connections prior to the Right channel going out?"

Yes.  I was comparing the pre + amp combo vs integrated amp.  Switching cables a number of times.

Currently I am experiencing intermittent right channel in/out when cables are correctly connected.  I have reversed connections several times.

I recently had a channel suddenly go out on an EAR MC-4 SUT. It turns out the culprit was the left output RCA jack! Upon opening that jack up, tech found a lot of extra solder cruft that was causing it to intermittently short out. Rare, but it happens. Anyways, I isolated the issue to this one jack by wiggling various RCA connectors while playing music at low volume. 

Since you have MM and MC input jacks, I’d try both. See if you can isolate it a bit more to either the MC SUT or RCA input jacks. Putting an MC cart into MM inputs, or vice versa, isn’t great but should be adequate for a quick test.

Also while the unit is on, I’d look to see if all the phono stage tubes’ heaters are at least lit up (careful not to touch anything inside the box).

The cart MM, no need for MC musings. To diagnose you should try the stand alone first after cables.