How would more voltage make the LED go off?
One LED (Green LED1) shares current with two relays, so the voltage is naturally dropped before the LED. I don't know from channels, can we refer to the LEDs as LED1 and LED2 as on the schematic? Or at least RED and GREEN, which it has printed right next to them.
If the red LED2 stays dimly lit, then either the transistor that drives it is leaky, or it is not being turned off fully. That is TR1. There is the panel button for channel switch, if it gets dirty you could have this. Also the Footswitch jack has cutout contacts, and if they get dirty, same thing. SO before we get too deep, clean and check teh FS jack and panel switch. MEASURE resistance across their contacts when closed. It SHOULD measure less than an ohm.