Open Baffle. Why are they mostly limited to DIY?

The Gradient OB's have been available for some time! I recall they made an OB woofer for use with Quad ESL's.

Correct roberjerman, Gradient made dedicated OB/Dipole subs for both the Quad ESL63 and original ("57"), which doubled as stands for the speakers. The GR Research/Rythmik is similar in basic design and style, but quite a bit more advanced. Better designed H-frames, higher-quality drivers, and more powerful plate amp. The amp also contains not only Brian Ding’s patented Direct Servo-Feedback system, but also a dipole-cancellation compensation shelving circuit, which corrects the front-to-back cancellation (and resulting loss of output with declining frequency) endemic to all dipole speakers. A brilliant design!

A pity Gradient did not design one to work with the more modern Quads. They still do a standalone generic one, but it is huge and expensive.

Gradient needn't bother. The GR Research/Rythmik OB/Dipole Sub is far superior!

Kenny, your system looks incredible, congratulations. Nicely done.