What to use as a subwoofer amp

My original plan was to use two 214's bridged like you, but tech at NAD said it was a bad idea since the drivers are 4 ohm. I guess thay were right.
I'm running the 218 amp in stereo mode, parallel. The amp has only clipped once, and that was me trying to bring the house down. It doesn't seem to have any trouble under normal conditions, even under very loud conditions. It sounds like yours didn't give you any trouble at first, but over time it wore down the amp. I asked NAD if that would happen with the 218 in stereo,and they said it wouldn't be a problem, only if I bridged it. (this would be 1000 watts@4 ohms which is too much anyway)

Having said that, I guess my question is if a class d amp will give me more slam. Or, if a class d isn't as good as the 218, is there an amp in the $400 to $500 range which is better suited for bass?

look at a pro amp--qsc,crown

I would go for a Bryston.

Bryston is a great choice, because you can easily bridge it and get the most out of your amp. I use an old 4B for my passive sub.

I use an old Acoustat Transnova amp on my subs and it works great. Hafler makes more current versions of this amp that can be found fairly cheap. A 350W/Ch version went unsold on Ebay this week for $750.

Good luck.