Squeezebox Classic issue

I've just bought a new amp to fix a problem that now appears to be my old SB Classic playing up. Luckily I have a spare Classic.
The problem is that the output on one channel is very quiet compared with the other. It plays, but at greatly reduced volume.
I've plugged it onto a different setup & get the same result. Swapping output leads swaps the channel with the fault.
It's not an LMS issue as the new player fixed the problem.
Any ideas please?