funny - I have the same problem since the same day (no joke, it was friday), as I replaced my wireless connection of the subs to cable connects (rca). There is hum, altough no RCA connected - same as you. But it´s another sub manufacturer (also 2 pin connector for power). I figured out, that when I unplugg some devices, plugged in at another wall socket (but the same main fuse), the hum disappears or gets much lower in noise. In fact in my case it was the wall plug of a minidsp. My solution was to turn the amplifier in the subs as low as possible and the gain in the preamp as high as possible, so it´s not that audible anymore...