This is what I think. If you have listened to the RBH speakers and you love the sound, then it makes sense to get the SVT Towers (SV831/SV1212N).
Since my first HT system (NHT SuperZero & SW2P Sub) 25 years ago, I have always preferred the use of an external amps for my subwoofers. It just makes sense to me. I've heard many stories about internal subwoofer amps malfunctioning over the years. So passive subs + external amps is my style.
I also loved the passive Funk 18.0 subwoofers, which I also used external amp to power (ATI AT3005 amp) and Denon Audyssey to EQ the Sub.
I have also used Velodyne passive Subs that were powered by a Dayton/Parts Express Subwoofer amp (Denon Audyssey to EQ).
If you are not buying the RBH SV831, then any subwoofer in this class should work great.