Well, if you're not using shielded interconnects, maybe you should try that, or go to RadioShack and get some RFI filters to install on the interconnects and power cords. Anyway, that's where I'd start. The longer the cable run, the more likely it could be acting as an antenna.
The tube gear may be more prone to picking up RFI than your solid-state components.
The tube gear may be more prone to picking up RFI than your solid-state components.