Strange Issue with NAD C372, I think

I have an NADC372 and I was listening to my Cambridge Audio 840c cd player yesterday and I noticed that vocals seemed to be coming from the left side too much. After investigating, it seems like the left speaker is a few db louder in the midrange. I switched the output cables of the cd player and the symptom switched to the right side.

I then tried my P3 turntable. This time, the extra loudness in the midrange was on the right side, with possibly more treble too on that side. I switched the output cables on my Rega Fono, and now the symptom is on the left side.

Does anybody have any idea what is going on here? It started all of a sudden yesterday. The c372 is about 6 years old, with probably 10 hours a week use.