I have the Sierra 2 and horizon center paired with a SVS PB-2000 sub. My next upgrade is for another sub. Do I go with the same model sub or do I mix it up and get a Rythmik sub?

The room is a dedicated theater. 19' X 15' with 8' ceiling. I am currently running 7.1 setup with SVS prime satellites for the side and back surrounds.

Thanks for any input!