sub troubles

I currently have a 6+ year old Sony SA WM-250 "sub"woofer. It is modest but more that enough for my current hometheater needs. My problem is when I turn the sub on it begins a very loud, low, hum starting any where between 20 minutes and 1 hour after turning it on but not immediately. It will do this with and without the rca cable connected leading me to believe it is internal to the sub. I opened the back and see no signs of leaking or bulging capacitors. What else should I look for?

Just in case it matters:
Reciever: Denon AVR-591
Sub: Sony SA WM-250
Center: PSB C1
Fronts: PSB B1
Rear: PSB A/V

I would be very tempted to replace the capacitors in the power filter. They can be dead without looking it.
That's where I'd start, though I'm not an audio guy.

Could you post a high res pic of the PCB components?

Six years old would tell me that it is a good possibility that it needs some caps. I have a 6 1/2 year old monitor sitting here waiting to be recapped.

The first two pictures are of the main amp board and the last picture is of the input/gain board. I assume the two big ones are the power filters. Right now it looks like they are siliconed to the PCB is that just to keep it for breaking the leads?

*edit* the green vertical board is just the transformer.

They look ok in the picture. Do you see any caps that are bulged or appear to be leaking?

None of them look to be bad.