I ran across the same posting for this sub as you and in doing research about it led me here. Small world. I believe he has a PC12-NSD sub with a sta-400D amp. I'm just assuming this of course. I did find this (link below) and it sounds like a similar problem. If it is the same issue the driver alone is $249. Hope this helps. Seems too much in the end for a sub that has already had the board replaced(amp I'm guessing). This thing must have been driven at high levels and a lot. Good luck.
I have had the pc12nsd for almost 30 days, set-up was easy as it gets. Sound quality is above its price range, deep and powerful. Only problem is I turned on my system only to find an odd rattle coming from the sub. I thought i was hearing things at first, so I walked to the sub and sure enough. My new sub was making an internal rattle every time bass came through it. I reached under and touched the woofer and the noise would go away, and continued when I took my hand off. I must say I am VERY disappointed as I have a Rel Q201e sub for the last 12+ years without issues. I called SVS on a Sunday (impressive they answered) and they said it sounded like a defective unit. I am very concerned about the quality of this sub now. I do realize this can happen but my equipment gets sold before it goes bad as I only buy high quality home theater components. So SVS handled the situation as good as they could have offering to replace with new, send me a new woofer or refund my purchase. I chose the placement sub since I did not want to replace a woofer on a 30 day old unit and I sure did not want to start my search for a new sub all over again as I spent much time researching a sub to replace my Rel that still works just wanted more low bass output.To SVS credit I asked if I needed to upgrade to the Plus, thinking maybe I over drove the sub. Doug said not necessary that the amps protection would have prevented damage and just sounded like woofer separated internally and very unusual. I have packed it up in oe box and my new sub is on its way already, I hope I have better luck with this one.
Disclaimer: The words above are based on loose facts mixed with my opinion, the latter of which is subject to change.