Subwoofer options for Magnepan with crossover betwen pre and amp?

Brian's quite a guy. Danny too. Brian found the OB/Dipole sub to be too lean sounding, without the weight he expects from a sub. But that leanness is one reason it works so well with dipole speakers. And their dipole figure-of-8 radiation characteristic matches that of dipole speakers, one reason for that leanness. OB's excite fewer room modes than do omnipoles, resulting in less "room boom", the "overhang" in bass notes some people blame on subs when the real cause is the room itself. Without the low bass reproduced by a sub, the room won't have it's lowest modes excited by the loudspeaker (which doesn't produce very low bass), hence the room's ability and tendency to boom remains unrealized. Add a sub, and that ability and tendency becomes apparent and audible. Fat, "slow" bass (overhang) is always blamed on the sub itself, whether justified or not. 

I'm about to set up both my OB/Dipoles and Magneplanar Tympani T-IV's in my new room---I'm interested to see the similarities and differences between the OB sub and the T-IV bass panels.

Your in Austin, ay limniscate? Check out an ol’ pal of mine from San Jose, Cornell Hurd. He fronts a Western Swing Band, and plays around town regularly. Cornell’s a great songwriter---Junior Brown for one has recorded a song of his.

Regarding calling Rythmik, for the OB/Dipole sub you want to call Danny Richie at GR Research. He and Brian Ding at Rythmik collaborated on the sub, but it’s Danny’s baby. He’ll tell you all about it; Brian Ding sells the DIY kit, but recommends his own sealed and ported boxed subs in preference to the OB/Dipole. You probably already know the G in your F12G refers to GR Research. The G woofer has a paper cone, the standard F12 an aluminum one. Danny feels the paper-cone woofer provides more resolution and a more natural timbre than does the aluminum.

Here’s a post Paul McGowan made on the PS Audio website, in one of his blogs:

"There are many great subwoofers available today. Perhaps my favorite is REL out of England. But even as good as REL subs are, they don’t quite live up to the Nth degree afforded by a properly designed servo version. That’s always been a mystery to me, why servo subs aren’t more prevalent".

Having owned the Infinity RS-Ib (six 8" servo-feedback controlled woofers per channel), and now owning a pair of Rythmik F15HP sealed subs and a pair of GR Research/Rythmik OB/Dipole Subs (both of which include the Rythmik Direct Servo-Feedback circuit), it is to me as well.