Yes, sort of. What was happening (and, doesn't happen anymore however much I try) was that RL1 was flipping into the 'energized' position which switches the Return on. Because it was only doing so for a split second, the signal (entering the front of the amp) wasn't actually dying in time for it to become very apparent.
the switch itself, seems absolutely fine.
the resistors involved. In circuit, but the voltage drop and measured resistances remain stable.
the smaller electrolytic caps in the middle of the board and to the right around IC2 (TL072CP buffer). All dead on spec.
the solder joints for all components involved by optical inspection, chopsticking and even lightly bending the board.
the voltages on the relays in rest and energized states - stable and fine, certainly didn't witness any current leaking in while the FX loop is set to off.
the cable/wires between the mainboard (CN18) and the board for the FX loop, switch and DI out. No fault found.
the flyback diodes (D13, D14) for the relays - same forward voltage, both measure correctly.
the voltages on the secondaries of the PT. I'm not sure what they should be as I'm lacking a datasheet for the transformer and the heaters seem a bit high, but stable.
I'm at a loss right now, honestly. When/if the problem rears its head again, I'll see what I can do...