@gnjtack. you are SO right about the OB sub and dipole speaker pairing being a great one. There are a number of us Maggie/Eminent Technology/Quad speaker owners using a pair of the GR Research/Rythmik OB/Dipole Servo-Feedback Subs with them. But as you said, it is a DIY product, the woofers and dedicated servo amp available as a kit only. However, there are a few woodworkers making flat pack OB frames to install the parts in, the flat packs pretty easily assembled.
Linkwitz Lab offers it's own OB/Dipole sub as part of complete OB speakers systems, but again as a kit only. The top model is tri-amped, the consumer required to provide three stereo amps (or six mono), including the one for the woofers. That can get expensive!