Good idea on reversing cables! No, not sure that there is not a tube for L/R channels, but from the schematics I found online it was not obvious - yes, it has 3 tubes. Ya, I thought that it could also be cartridge or cable. Started by checking amp using digital source, which was fine, so problem is upstream. Running a VPI Scout turntable with 10" JMW Memorial tonearm and an AMG Teatro MC cartridge (have a Transfiguration Axia as well). The EAR feeds into a Line Magnetic 211 integrated amp.
I will try reversing L/R channel cables from EAR to IA, and from phono to EAR. That should isolate the component.