Sound like a radio I used to play underwater....
Actually...a capacitor installed at each speaker connection are fantastic filters. Have installed them on eq. during the golden age of cb's to prevent bleeding into TV's and other sound systems....not sure what values to use however.Even the most expensive sound systems of today uses cap's at filters. If HD can't fix it give it a try..about a $ 10.00 investment. Are of course you can upgrade the system...lol