Very, very few @jim2. In regard to the servo-feedback subs of Rythmik Audio it is because RA is a direct sales (to consumers) company, so being able to hear a RA sub before purchase takes some work. The same is true for GR Research, with the additional hurdle of that company offering their subs (and loudspeakers) as DIY kits only.
I’ve posted here on Audiogon about the Rythmik Audio/GR Research servo-feedback/open baffle/dipole sub (the sub is the result of a collaboration between Rythmik’s Brian Ding and GRR’s Danny Richie) on numerous occasions, and the number of Audiogon participants who have heard that extremely unique sub can probably be counted on one hand.
The world of subwoofers can be divided into three categories:
1- The one-of-a-kind Rythmik Audio/GR Research OB/Dipole Sub. The only people who don’t know how different this sub sounds from all others (including the other models from both company’s) are those who have not heard it. That includes almost everyone reading this.
Think about it: servo-feedback control of woofer excursion, dipole propagation (with a plate amp that includes a dipole cancellation compensation circuit), and open baffle sound characteristics. Wow.
2- Another one-of-a-kind sub, the Eminent Technology TWR-17, the world’s only "true" subwoofer. Designed to reproduce only 20Hz down to 1Hz!
3- All other subs.