artemus, for clarification here's some additional info: Rythmik Audio and GR Research have a unique working relationship and marketing arrangement. Brian Ding and Danny Richie are both located in Texas, and pooled their talents to design some of their products.
Brian Ding of Rythmik is the designer (and Patent holder) of his Direct Servo-Feedback circuit, which is included in all versions of his plate amps. He also designed and builds all the Rythmik factory-finished sealed and ported subs, also designing and having built his aluminum-coned woofers (as well as the paper-coned woofers found in his budget subs).
Danny Richie of GR Research is a speaker designer, focusing on drivers and x-overs for open baffle loudspeakers, for the DIY speaker builder. He offers kits only, no finished loudspeakers in cabinets, though he does have a few guys making enclosures for GR Research customers, built to Danny's specs. Danny is responsible for the paper-coned 12" woofer offered in the Rythmik F12G (he prefers the sound of a paper cone, and feels his provides low-level resolution and musical timbre superior to the aluminum version), the non-G version having an aluminum woofer.
Brian and Danny both sell the Rythmik DIY sub kits, while Brian is the marketer for the finished subs. The sub flat pack enclosures offered by both DIY Sound Group and Parts Express are one and the same, PE buying them from DSG, marking them up a little, and reselling them. There is both a 3cu.ft version and a 4cu.ft. one, the larger intended for 18" woofers, but recommended for the Rythmik 15" woofer as well.
The DIY Sound Group flat pack enclosure is an okay design, but could be better. Adding more internal bracing is a real good idea (Jim Salk's enclosures for the Rythmik subs are absolutely incredible!), as is adding a second layer of MDF to the single 3/4" panels, which are barely sufficient. I designed my own 4cu.ft. double-walled sealed enclosure, drawing up the plans and having a flat pack cut by a cabinet maker in my locale. Assembly with wood glue and braces is not too difficult, and the MDF can be veneered or painted in any way one wishes. I chose "Hot Rod Black" by John Deere, intended for their farm tractors. Looks real cool, like a 50's custom car---not glossy, not flat, more of an eggshell type low sheen. No flames, though!