Rythmik's claim to fame is using great drivers along with direct servo to really reduce all sorts of distortion. They make vented and sealed subs, and also offer DIY kits.
SVS uses DSP and great drivers to have great performing subs. They also make vented and sealed subs.
Funkywaves uses some really great drivers in custom enclosures with top notch build quality / fit and finish. You can have FW make you a vented or sealed sub, a passive radiator, maybe even bandpass, transmission line, or folded horn subs. It's 100% custom. Passive Radiator subs like their FW15.3 have an advantage over ported subs in that there is no possibility of creating pipe resonances or vent noise - you can actually squeeze a bit more low end output out of them. TLS Guy over here could probably tell you why he thinks classic transmission lines are great options when it comes to sound quality.
I think all of the above are great choices for high end subwoofers. Funkywaves is a great option for canadians like us because there's no extra duties to pay. SVS through sonic boom audio is marked up quite a bit (which is understandable). Rythmiks must be ordered from USA.
Funkywaves are local and use great drivers, and can be custom designed with YOUR input and YOUR choice of finish.
Here is the Audioholics FW thread. - scroll down to post 21 to see warpdrv's bedroom FW sub.