Let's talk SUBS

Currently my set up is as follows
B&W 802 fronts
B&W Htm1 center
B&W 804S rears
Anthem statement p5 amp
marantz 8801 processor
PS3 blu ray
Sony vplvw60 projector
Stewart film screen 110" screen

25' by 25' by 8' ceiling
50% drywall 50% stucco
commercial carpet
acoustic panels at the first reflection points.

For my sub i have an older paradigm reference servo 15.
i really enjoy it, a big.upgrade from my older vega 15.

i can run two subs from the marantz processor.
it is hard to find a matching paradigm because it is old.
so what would be a good sub to match up with it?

An older used sub would be fine as well, it doesn't have to be new.
i want great bang for my buck. ie the paradigm i got used for $300... i know it is not the best sub out there BUT, it would be hard to find ANYTHING better for that price.

Thank you for your suggestions,
Bill