GCA-250 amp cutting out only in phono mode

Paired with PS Audio PCA-2 preamp w/outboard power supply. Any ideas on why this might be happening? All RCA cables and ground wire seem to be OK. Something in the turntable [Music Hall MMF 9]? Thanks for any ideas.

Makes me think it’s something upstream. The GCA won’t really care where the signal is coming from.

unless it’s extreme LF from the record deck inaudibly overloading the amp?