I would think a AC line conditioner should work, or perhaps an UPS?

My thinking is that it is an active circuit inside and should prevent any 'feedback' being fed back out (the output should be isolated from the input) - but on the other side, it should also take the incoming 'bad' signal and clean it up for use by your speakers. But I am just making a 'guess'.

Just a shot in the dark, I would be interested in knowing the actual results and if it helped.

edit: Just wondering if you tried a simple surge protector on the pin? I doubt that it would work, but never know.

All video games had an isolation transformer wether they are stand alone or built into the monitor chassis (as did a lot of older electronics) - has nothing to do with 'noise prevention' but more to do with electrical isolation as the frame of the monitor (etc.) is tied to the hot side of power (vs ground).

An isolation transformer wouldn't work as it would still transfer the 'noise' spikes back through the main windings. You need an active circuit that isolates the 2 halves - with an active circuit power would only flow in 1 direction any returning noise would be stopped. A line conditioner should do that as it takes the original power feed and breaks it down and reforms it into clean power before sending it out - it should prevent any returning noise from passing though to the main feed.