I'm stumped to get any logical answer to this as well. I've never used anything other that the phono's for recording so it's definately something that's turned up recently.

Looking at the PCB there seem to be a couple of surface mount transistors and capacitors in the circuit to the phono's but I can't see how a single-point component fault would affect both channels, and even then I don't see what sort of fault would give such an odd waveform. Hairline crack in PCB earth plane somewhere? I looked hard but couldn't see anything remotely suspicious. Oh well.