Experience has taught me that the best bass doesn't always come from a subwoofer. There are still many subs that don't match the accuracy of a good woofer. My Rythmik subs restored my faith in subwoofers.
Admitting to bias
I should admit up front that I can't be completely unbiased when talking about Rythmik audio. Back in 2006 I suspected that they had a great product backed by very sound theory, and proposed a website redesign. I had to hear them for myself, and I also wanted to know if they really were as good at the website asserts. It didn't take me long to verify that they really are exceptional.
It didn't take long to realise I was listening to accurate subs. The absence of distortion was instantly recognizable and the sound was dry from the start. The sound reminded me of the sound of some Focal woofers I'd heard at Aranmah acoustics. The cost of those woofers is about the same as buying the Rythmik driver and amp. Not bad considering the Rythmik has greater extension and output.
Set up and calibration
Set up correctly, the Rythmik subs are magic. But like any subwoofer, it's possible to set them up so that they sound terrible. In my system I use Behringer Ultracurve with a Behringer ECM mic for EQ. I use it to shape the response and this includes the bass response. It also helps determining suitable sub locations. In my room placement isn't critical - the room has a lot of bass damping, which means most practical locations for subs work well.
I use an 80 Hz crossover point and a 4th order electrical high pass supplied by the plate amp.
An earlier version of a Rythmik sub has been independently measured at the Home Theatre Shack:
HT shack Rythmik measurements
One aspect clearly shown in the measurements is that the distortion is low at all drive levels within the range required for music. The distortion below 40 Hz climbs fairly high, but this is mostly related to more artificial home theatre LFE use, for which the ear often has no real life reference. Subjectively the Rythmik subs are very clean and dry.
My previous subs sounded better in a H frame dipole arrangement, but I was surprised to find that the Rythmik subs sound just as accurate in a sealed box. I believe the reason is related to the higher level of damping control provided by the servo which eliminates box coloration in a similar way to infinite baffle and dipole subs. I've written more about this in my post about my open baffle speakers.
I have no hesitation in recommending these subs to anyone for serious home theatre and music listening. They don't use the highest excursion drivers or the most powerful amps, but I believe they operate in a sweet spot. Where more output is desired, I suggest multiple subwoofers, which is actually better anyway in terms of room acoustics.
Rythmik Audio Website