To OP Steve: Given that the problem may turn out to be in the relay, as pointed out by N8Nagle earlier, there may be a way to test/side-step the issue because this preamp has "main" and "bypass" outputs.
Looking at the schematic, there are two relays, RY901 and RY902, located near the main and bypass output jacks on the back panel. RY901 is for the main L and R outputs, and RY902 is for the bypass L and R outputs. In case you don't know, the bypass outputs are still controlled by Volume and Balance, but bypass the tone and filter controls. What is important here is that the main and bypass outputs are controlled by these two separate relays, and it is not very likely that if one relay is bad/sticking on one channel, that the other has the same problem. Also useful to note that the headphones come off the circuit for the main outputs, so they would show whatever problems those outputs show, which is what you found.
So if you have been using the main outputs to connect to the amp, try connecting the bypass outputs to the amp instead. If the problem disappears, it is caused by either the relay on the main outputs, or possibly a broken signal path in the tone/filter part of the circuit.
