Best subwoofer to pair with Vandersteen Treo CTs??

I really enjoy my Vandersteen Treo CTs and while the bass is good and goes down to 35 Hz or so, I'm missing the bass I used to hear when I had a subwoofer with a prior system. My listening space is 286 square feet with a 14-foot cathedral ceiling at peak, and it only has a partial wall separating the space from a kitchen and dining room (all open) behind the speakers with considerable space on either side of the speakers (one side opens into a hallway). I've been considering REL subs and the Vandersteen Sub Three, but older threads have also suggested Rhythmik and others. REL's "subwoofer finder" suggests the S/812 but I've heard others say a REL S/510 would be sufficient. I will appreciate any opinions you may have that will help ease my decision-making.