Thanks everyone for the ideas... Sidssp - I set it up and didn't change the VTF until after I noticed the problem.
I forgot to mention that I brought it back to Magnolia to test it there, and had the same problem, going through, I believe, a Denon that had a phono in.
So this seems to narrow it down to two possibilities: I set it up wrong, which seems unlikely since it worked well for a couple weeks. Plus it was in a secure place and not jostled at all. The other is maybe some sort of internal problem not of my doing.
I'm going to have to buy a preamp anyway, as I'm pretty sure that the Jolda 102B that I'm getting soon will need it.
My other option that I'm going to try is to dig out a cheap USB table I have in the closet to possibly eliminate the amp as the problem.
Thanks again for the suggestions.