Kenny, Danny and Brian worked on the OB/Dipole Sub together, Danny instigating the collaboration. Danny is an Open Baffle enthusiast, Brian not so much. When Brian heard the sub, he has said he found it to sound somewhat "lean", without the weight and fullness one expects from a sub. That leanness is one thing that makes it so special, and to work so well with highly transparent loudspeakers, which themselves sound lean (in comparison to the thick, slightly plump sound of so many box speakers).
In the "regular" Rythmik Sub line-up, Danny had input on only one model---the F12G, the "G" signifying GR Research, Danny’s company. The Rythmik F12 comes in two versions: the standard F12 (with a Rythmik 12" aluminum cone driver), and the aforementioned F12G (with a GR Research 12" paper cone driver). Danny feels the paper cone affords the F12G greater low-level resolution than does the F12’s aluminum cone (due to the lower mass of paper versus aluminum), and a more natural tonal balance (due to the resonant signatures of paper and aluminum). He recommends the F12G for a music-only system, the standard F12 only for use in a system in which it will be played at near maximum output, where the aluminum cone’s greater stiffness will be an advantage.
There are other Rythmik models with cones of paper, but they are budget models, not the top line F15 and F25 (dual 15’s). The difference between the F12 and the F15HP is, as you might expect, the 12" driver versus the 15" (greater cone area of course, but also greater maximum voice coil excursion), plus amplifier power---the F12 having a 370w amp, the F15HP a 600w one. The difference in price between the F12 and F15 is nominal; if I were getting a pair of sealed Rythmiks (which are slightly superior to the ported in terms of group delay and deepest frequency response), I would get a pair of F15HP’s.
In fact, I did! In addition to the OB/Dipoles, of course. But I bought mine as DIY kits, designing my own enclosures, buying the sheets of MDF, and having a cabinet maker cut them into flat packs which I then myself assembled. I made the internal volume of the enclosures 4cu.ft. vs. Rythmik’s 3cu.ft. (for greater efficiency and maximum output), and braced the hell out of them---1.5" Baltic Birch braces every 6", top-to-bottom, side-to-side, and from around the driver on the baffle to the rear panel. I also designed the enclosure as a box-within-a-box, leaving 1/2" between the inner and outer (with "ribs" supporting the inner), filling the 1/2" cavity with sand. Oh, they’re very quiet enclosures, almost non-resonant. And they weigh a ton! What price excellence? ;-)