Help - sub auto-on circuit not working!

I have a feeling that what I am going to ask is going to be too general, but I need to know the solution to this very annoying problem and figure this is probably the best place to start. So here it goes...

A year or two ago, I bought a used sub amp off of eBay because I had acquired a used Infinity Sub and had an extra box laying around, so I thought I would make a new HT sub. The guy that I bought it from mentioned that he had done a little bit of soldering work on it, but the only problem was that now the power light would be red when the sub was on and green when it is off. Well, I found out differently.

The auto-on feature of the sub does not seem to be working. Basically, if I want the sub to work, I have to unplug it, plug it back in, and set the power switch to "On" or "Auto" for it to kick in. The sub is supposed to turn on automatically when it receives low frequencies and it does not. The power light turns on (red) when it receives low frequencies, but the sub itself does not start kicking unless I do the unplugging technique. After that, it works just fine, but I am sick of having to unplug/plug the sub back in every time I want to watch something after the sub hasn't been on for a few hours.

Is there something basic in the auto-on circuit that causes this problem? Is this a common problem? Is there a general way to fix this? Like I said, I'm assuming that the answer is "No", but I was hoping that someone here would be able to help me figure out the problem.

Thanks in advance!