I think I can safely rule out the phono stage. I tried the MM inputs and changing all six tubes and experienced the same problem.
Next up is changing the cartridge - which will take me the better part of Saturday.
I checked both LPs (Barber and Cassidy) during system warm-up last night and they sailed through without a hitch. Sounds like the cartridge or setup is the probable issue. Since you have a new cartridge on the way it should help to identify the cause. there are already some good suggestions listed earlier in the thread. If you run into any issues, I'm sure the members here (including myself) will be wiling to offer more advice.
BTW: What cartridge did you order?
Dre