I can't find any specs nor reference to that model subwoofer using Google, perhaps I'm just not good at searching.
The reason I wanted to look is because you've got the very best in-wall speakers made, they would really shine if paired with a single (or better, a pair) of new subs that are commensurate with the quality of the speakers. A pair of 500w ported subs (or higher power) would give you a better chance at even distribution of low frequencies present in movies today. With a single sub of less power, you may be unable to get good bass response from all seats and/or not be able to reach reference levels or the true lowest frequencies.
Is the B&K 600DF+ still available? What amp does it have and is a sealed downfiring sub or ported front-firing sub?
I'm not trying to spend your money, just want to help assure that you get the very best results for the investment you've made to this point.
KEF R500, R200C, R100, SVS SB2000 sub, NAD T758 receiver, Pro-Ject RPM1 TT w/ Ortofon 2M Red, Bluesound Vault2, Sony BDP-S480 BlueRay player, Samsung UN55JU6500F 4K LED TV, KEF M500 headphones, Sony CDP-CE500 CD changer.