passive sub recomendations?

[jdc115]

I have an old M&K sub but the drivers need to be replaced which are quite hard to find at the moment plus I would have to get them shipped overseas so maybe not so cheaper either.

since I have a spare Parasound power amp (125 watts/channel), i could use the sub outs on my processor to feed this power amp to a passive sub. I am thinking this might give me a more economical, higher quality sub if I could find a good passive subs.

Does anybody have suggestions or used any passive subs with an external power amp?

Thanks for any recommendations, would like to find something under $500 but will always consider something higher if worth it.