subwoofer slight hum

I've tried and tried and just can't seem to find a solution so far.

It's not a big problem... I mean I have to get kinda close to the sub(s) to hear it, but there it is: a subtle but distinct hum down there.  

It used to be annoying with a throbbing pulse hum when watching TV or dvds, noticeable.  I solved that by plugging the subwoofer into a separate house circuit, with an extension cord.  I think that throbbing had to do with the "old" plasma TV drawing cyclically from the same outlet or some magnetic interference, I dunno I'm not an electrical engineer... (but it's the Panasonic ZT-60, which I still adore and won't part with soon).  Anyway, thankfully, the throbbing went away when I put the sub on a different outlet.  

But the hum down low still exists, both on the main listening and viewing room, and on another sub in a totally different part of the house.  

I've tried different cords, different surge protecting power strips including recently the Tripp Light Isobars (which I actually kinda like) but even they didn't stop the hum. 

I can live with it, I mean I can't hear it with even quiet music playing, but short of spending $1000 on some power conditioner or other, which from what I understand have mixed results at best (though I suppose that depends on the individual situation)... any ideas?  Might a $400 power conditioner do the trick, or is there some even less expensive possibility that I'm overlooking?