There is a lot of RF noise coming from any spark lighter like this. It may just be airborne and not coming from anything wired into the set. The first thing you need to do is figure out where it is getting injected into the signal path. I would disconnect the speakers and try a headset with a shielded lead to eliminate the idea it is coming directly into the speakers, although that is unlikely. Does it seem to favor any particular input? You could make up terminator plugs for any unused ones. If not you may just have it being "detected:" by a diode in the amp and I am not sure what you do about that other than better shielding around the amp.
I doubt an isolation transformer will do anything. If this is a fairly new amp, it has an inverter power supply.
