f shorting the rca cures it, I'll bet it'll be internal wire dressing as a quick google seems to show it's a common problem.
I haven't tried anything like that yet, thanks.Could you connect a chassis screw on the monitors to a chassis screw on your source/pre? I'm wondering if earth related?
I know active subs* sometimes hum if not earthed properly (*in particular Rel connected via hi-level).