What to use as a subwoofer amp


I am currently using a nad 218 to drive two SVS+ 20-39's. Do you think I am getting the most out of the subs using this amp. I know sub amps are different, but is that because manufacturers want to put everything within the sub, or are the class D and other types better for sub applications.
michaelscott73
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.
Because of their wide pro use and good reputations, I did consider the Crown and Hafler 9505 Transnova amps as well.