The standard solutions are a filter for the sub cable and/or a filter for the AC receptacle. I've tried the filter for the AC receptacle and it didn't do anything. I haven't tried the filter for the sub cable because I don't know how exactly it works. If it's a 60Hz notch filter, no thanks as 60Hz and the band around it is contained in music after all.

Of course the other solution is running the sub cable in high magnetic permeability conduit but I'm not going there .