I have somewhat-DIY subs. Speaker/cross-over designer Danny Richie of GR Research collaborated with Brian Ding of Rythmik in the design of an OB/Dipole subwoofer. The Rythmik Direct Servo Feedback A370 plate amp is mated with either two or three 8" or 12" GR Research servo-controlled woofers that have been optimized for open baffle use. Those components are obtained from either Danny or Brian, and the woofers are installed in a DIY OB "frame" of either "W" or "H" structure. Plans for the frames are available on the GR Research website, and can be fairly easily made (or have built by a cabinet maker), as well as being available as a flat pack by a couple of people. I got lucky and found a guy selling a pair of W-frames made by ED before they went out of business.
If a dipole sub sounds familiar, it's because the Finnish company Gradient offered such a sub, made and marketed specifically for use with the QUAD ESL63 in the 80's and 90's (check your old TAS' for a review of it). Gradient built their H-frame with the same footprint as the Quad, being designed and intended to be used as a base for the Quad. The GR Research OB sub is quite a bit more sophisticated, and a much-higher performing one.
The woofers are installed, in the case of a 2-woofer sub, with the drivers facing in opposite directions, wired in opposing polarity. The open baffle operation results in a null at each side of the frame, as in any dipole. This causes increasing roll-off as frequency descends, with which to compensate for Brian designed and installs into the amp a shelving circuit.
The sound of the OB Sub is very different from any and all other subs. The servo-controlled woofers, combined with the lack of a resonant sealed or ported enclosure, results in a very "nimble", start-and-stop-on-a-dime sound quality. The sub doesn't make it's presence overtly known, instead making the speaker it is used with appear to have gained an octave or more of extension at the bottom. It doesn't have the thick, heavy, slightly-behind-the-loudspeakers sound of non-OB subs, even very good ones. I know you guys with really good subs (including Rythmik's "regular" ones) think yours are "quick", but you won't after hearing the OB/Dipole Sub! Just ask anyone who heard it at RMAF (either in free-standing form, or as part of the GR Research Super-V loudspeaker), where it was voted Best Bass at the Show many years running.
While being a great sub for any application, the OB/Dipole is, for obvious reasons, particularly good for use with any and all dipole loudspeakers. If you remember, the SPL decrease with distance differs between a dipole and a non-dipole loudspeaker, and the same is true with subs. If you mate a dipole speaker with a non-dipole sub, the balance between the two will differ according to location and listening distance---not good. With a dipole sub, the balance between it and a dipole loudspeaker remains constant, regardless of where the speaker/sub and the listener(s) is/are located. The balance between the two will also be affected by their differing room-loading characteristics. Speaking of loading, the Dipole sub doesn't load the room with bass the way a non-dipole sub does, that loading resulting in the room boom familiar to subwoofer dislikers. But, then, what of the OB/Dipole sub for use with a non-dipole loudspeaker? True, the balance with listening distance will differ, but with most non-dipole loudspeakers that is far less of a problem than the speaker's inherent shortcomings. Okay, I'm a dipole lover!