Marbles, you drive a hard bargain. When I see a question like this, I want to take my time so I can provide an appropriate, thoughtful resonse. Then, a few months later when I finally find the time, I forget I was going to answer. So I'll take the time now to provide a few quick impressions before my memory fades.
I might point out, here, that we have only done a few Rythmik projects to date. Naturally, when we build them, customers would like them delivered rather than letting me take them home for an extended period. So my comments are based on rather limited time with the units we have built.
With that in mind, let me start by saying that I love the Rythmik drivers. It is natural that I should say this as they are obviously based on the original TC Sounds drivers (as far as I can tell) - drivers I have had a great deal of experience with and that I believe are some of the best deep bass drivers ever. In fact, Thilo may build these for Rythmik - I don't know. Rythmik simply adds an additional voice coil for use in a feedback loop with their servo amps.
When we built the first 15" Rythmik sub, we tested it against our SongSub. We set the crossover at about 80Hz and ran music and tones. The SongSub is a very low distortion sub and the two tracked very similarly to a point. As the bass went deeper, however, the Rythmik took over. Of course, it should have since it has more displacement - a 15" driver should move more air than a 12" driver, all else being equal (there is no substitute for displacement).
The nice thing was, this first Rythmik sub was sealed. So the bass should have rolled off much earlier than the SongSub and it did not. In this case, the servo forced the system to play deeper than it normally would. That is the good thing. The not so good thing is that the amp was almost wide open in order to get the same output as the SongSub with the gain set at about 30% of maximum. This is because the Rythmik amp is not as powerful in the first place and a good deal of power was consumed forcing the system to play deeper than a sealed system would normally play. But in the end, there was enough gain to mate well with a pair of stand mounted speakers. (I understand Brian is working on a higher power amp at this time and that will be a nice addition to the line.)
The sound quality was extremely good. With a little care, it was easy to set the system up to the point where you did not notice the sub was playing. The only way you could tell is that the system played much deeper than the stand-mounted speakers would have played on their own. That is the mark of a very good subwoofer - very tight, very accurate and very musical.
How would this system compare to a 15" driver in a cabinet with dual 15" passives and no servo control. I don't know, but it would be an interesting comparison. My gut instinct tells me that if the cabinet and passives were designed and tuned correctly, the results would be very similar. In that case, the servo would not be asked to do as much since the driver would play deep and fairly accurately on its own. But with the cost of the Rythmik solution, there really isn't any reason to test that hypothesis unless you need more power. The servo technology is just not that expensive to begin with so why not take advantage of it.
Now, to answer another question, why would we use passives? Well, to gain better overall performance, you would normally port the cabinet. This would extend the natural bass response. The problem is, when you try and play bass around 20Hz or so, port noise becomes a real issue. You could always increase the port diameter to deal with this, but with these drivers, the port length becomes EXTREMELY long EXTREMELY fast. It is simply not practical to try and cram an 80" long port into a cabinet that is already rather sizeable.
The solution is passive radiators. With the help of Jeff Bagby, we determined that two well-tuned 15" passives would provide the performance of a ported cabinet without any port noise at all. The system would play plenty deep without putting any exsessive demand on the amplifier. And I think the results worked out very well. The 15" Rythmik with dual 15" tuned passives required far less gain than the sealed version we originally played with. I was very pleased with the results even though I didn't have a great deal of time to play with the resulting sub (Josh was waiting for his speakers to ship).
As for my personal sub project, I am just putting the finishing touches on a new subwoofer system for my home theater. It consists of a pair of HT4 woofer cabinets tuned to 18Hz for in-room repsonse into the mid teens. For those not familiar with the design, these cabinets feature a 12" driver of the same type, combined with a pair of 12" passives. I will set it up with a Velodyne SMS1 and a pair of 1200 watt amps. I susect the results will be very good - servo or not. But we'll see...
I wish I had more time to organize my thoughts, but that was off the top of my head with no time for editing. All to make Marbles happy...