I had the same issue as you with this Monoprice 12" sub: it would hum loudly even with no inputs connected. However, I found a fix. This reply might be a bit late for you, but in case anyone else has this issue, here's what I did. Note: this fix involves opening the subwoofer amplifier, which could void the warranty, and exposes the user to live 120V electrical circuits. Only do this if you know what you're doing in a safe way. What I did was open the amplifier and re-position some of the cables. There are 3 small cables that run between the two circuits boards in this amplifier, and their physical placement affects how much RF interference they pick up, and thus how much hum/noise can be heard from the sub when it's turned on. In particular, the cable labeled "BASS" is very susceptible to picking up noise (it is the audio signal cable). I found that if it was close to the large transformer for the amplifier's power supply, and if it were at a particular angle, it picked up a lot of noise. So, I moved it around until I found a spot with minimal noise (I had to do this with the sub powered, so I could hear the hum change), then I hot-glued the cable to keep it there. Note: the 3 cables in my sub were tied together with a ty-rap, so I had to cut this so I could move each cable independently. One of the other cables, labeled "V+ V- GND" (which supplies power to one of the circuit boards), also required re-positioning to minimize noise. Note that when I moved/re-positioned the cables, I didn't unplug them: I only had to push them around a little bit to change the level of noise/hum significantly. There is a third cable connected between the two circuit boards in this amp, but I found that its physical position did not affect noise at all. My sub used to have a hum so loud it could be clearly heard from 20'+ away. Now, it's barely audible 3' away directly in front of it. As I said before though, opening this amplifier to do the fix I did can be dangerous, so be careful!