In the field of pro audio where I make my living, this comes up regularly. The trick is to give the RF a place to GO, besides the next stage. There are many ways active electronics can demodulate, downconvert, and heterodyne with RF emissions that get caught up in open circuitry. Also, some exotic cable designs give up shielding for the idea of supposed lower impact on the musical signal. Here are a couple of tricks I have in my bag:
Move your gear a few feet! RF energy pools like invisible currents of water, with nodes of dead spots and peaking stronger areas. There is a chance that multipath can work for your here, and you can find an arrangement that will kill the radio show, in a cancellation node (the fence board part of the "picket fence"). Moving cables around can help.
Try different cables. Start with a cable with a clear design goal of minimizing external influences, that has a solid shield, braided, with a drain maybe, or even foil and a drain. Stay away from twisted pairs and no shield. Unplug and plug in all of your connections; some patina/corrosion can act as a crystal set to demodulate AM broadcasts.
Buy clamp-on RF chokes. They come in several shapes and sizes, and can reduce the effectiveness of your cables as antennas. Try them on your power cord or input/output cables.
Buy some ceramic disc capacitors. If you know your way around high voltage circuits, bypass power supplies, high impedance stages, and inputs and outputs with values ranging from a few hundred pF to 0.1uF. The idea is to provide an alternate path for RF energy away from the circuit that is demodulating it, without impacting baseband audio below 20kHz. These parts should ALREADY be in your gear. There is a chance that a lead or solder joint on a cap has failed, as indicated by the fact that only one channel is affected. If you're not so sure what is running at 90V to 350V in your tube gear, don't mess with the insides (for your safety), just try the external connections.
Lastly, double check for tight screws on the chassis. Again, sometimes a little corrosion takes on the form of an oxide that acts a bit like a diode. Unscrew and retighten all fasteners that go to PC boards, ring terminals, and metal parts.
Good luck!
:)
