I'm the guy with the new sub in the works. Basically, Rythmik sells complete subs, and they also sell parts so you can build your own. These vary from "driver and amp" only kits, to everything included, but not assembled. For the guys who want to buy driver and amp only kits, they also have a section of their website with basic cabinet designs that will work, so you don't have to model it out yourself for the driver you just bought. In short, I had Jim buy the driver and amp from Rythmik, and I think he said he essentially follows their basic enclosure size. Brian Ding and Jeff Bagby have both had input into the design.
I was looking at a SongSub, but I wanted something with more oomph and a little higher end. I got to talking with Jim, and he went over all the different types of subs: sealed, ported, passive radiator, powered, unpowered with external amp, etc. Essentially, we both ended up with the same conclusion, that a servo sub with passive radiators would be one heck of a product. Unfortunately, at the time, we didn't think there was an amp with servo circuitry that would work in a passive radiator design, and I thought I would have to pick between sealed and ported.
I called up Brian Ding at Rythmik and asked if he had any ideas about such a servo controlled sub with passive radiators. He said it was definitely doable, and he had modeled it, and it works fine if you follow his plan for a ported sub. So I went with the 15" driver, and it was decided that two 15" passives would be great with it. Jim got the final ok from Jeff on the model maybe a week or two ago, and so the box is going to be 20"W x 20"D x28"H. He had a guesstimate of 120-130lbs using 1.5" thick walls, but we'll have to wait and see on how much it really weighs.
I have to say I'm really excited about this product. Its going to be a beast to move, but it should have a lot of output, a lot of sound quality, and no port noise. I would definitely give Jim a call, he can make you anything you want, this was a design that has characteristics I was looking for, but didn't even know could be built.