First off, it isn't your speakers I would guess..
I would imagine it is your amplifier that is responding.. the speakers are a byproduct.
Have you tried isolating the amp, moving it around? Are there any other sources by it to exacerbate the situation, like TV's, floro lights and things like that in addition you might be able to isolate.
But then you say 12v dc.. which leade me to believe they are powered computer speakers, and those should be fairly well shielded.
There are LINE CONDITIONERS ..
http://www.graniteaudio.com/zero/index.html
Or less expensive gadgets that claim to help
http://www.amazon.com/PS-Audio-Noise-Harvester-suppressor/dp/B000Q7SCWA
If it were me, I would be inclined to start 'flipping' other plugs in the house as well.. you might get lucky and clear some leeway in the circuit thereby suppressing some of the noise.
Good luck~!