I think the Rythmik vs JTR discussion might be better framed as how much output do you need? The JTR have more output (I'm not convinced on that much more extension, given the tuning frequencies and data from data-bass) but is that really necessary in 90% of listening spaces? Unless your music listening involves strong content below 25Hz at levels exceeding 105-110 dB, even the upper levels of Rythmik are more than you need (assuming you don't have a 6,000 cubic foot listening room).
The JTR subs cost quite a lot more than Rythmik and are not as nicely finished -- an F18, even without the gloss black, is more aesthetically pleasing in a multi-use listening room than any JTR sub.
In other words, I'm not slighting the performance of JTR at all. They are an amazing product, and priced as such. Even though some people do use them in multi-use rooms, I personally want something with a better appearance to go with the performance. I doubt I'm alone in that. So, for many people, I don't get the cost and aesthetic compromises being made when a less expensive, nicer looking product with sufficient output is available.
EDIT: a single F18 can generate 110 dB free-field above 30Hz...again, for primarily music listening, how much more does a 95th percentile person need?