I've got two amps that do it, and three that don't. My Thunderfunk 550B and SWR SM900 both pick up the local FM radio station quite loudly and it's annoying as h***. Also quite a bit of 'hashy' hiss. ONLY happens if an instrument cable is plugged into the amp. I've tried several cables with no change. DOES NOT MATTER if an instrument is plugged in or not. Moving/rearranging the instrument cable has a HUGE influence. certain arrangements nearly eliminate the noise. But why just these two amps? My others are silent. Pressing MUTE and turning input gain all the way down on the TF reduces but does NOT eliminate the noise. (!?) I think this is a clue but I don't know where to follow it. Ground is intact from cable sleeve to third prong on power cord. Outlet tester shows two amber lights (all good). Any ideas?