M&K Sub Dying...

[Xenomorph]

I'm afraid my faithful M&K MX150 sub is on its last legs. Lately I've noticed a huge loss of power coming from this sub. The drivers are moving but not putting out the bass it once was.

I've taken the whole sub apart looking for any type of problem and found nothing.
It's not the receiver, I've tweaked everything I could..even cranked up the signal to the sub.
I've swapped cables.
Opened up the sub and checked the amp itself. No blown or swollen caps, no fried fuses, no loose wires. All wires inside and to the drivers are good.
Even swapped the drivers themselves out with a couple of Daytons. Nada.

This thing used to shake my house and now it's barely putting out anything.

One thing and this has been an ongoing thing for years, the gain knob on the back needs to be turned back and forth a bit sometimes. If it's in a certain spot (well, several spots actually) there will be no bass at all and you need to wiggle it a bit.
Could it be a bad pot? Would that be the source of all the problems? If it is I'll kill myself because I've been lugging that heavy thing back and forth for 2 weeks looking for the problem.

Any ideas would be very much appreciated!