Sub Question. One higher quality sub or two lesser quality? (Rythmik vs HSU)

It’s been said a million times there is no substitute for quality

And always worth repeating. I would rather have 1 higher quality sub than multiple lower quality subs. Add another when finances allow if you think you need to.

Get two powered and connect from amp speaker outputs. Never again will I try to integrate one into a 2 channel music system. 

Thanks for all the replies so far. My inclination at this time is to go with one VTF-15H II with the thought of adding another (or more) if necessary. My room is pretty large and bass friendly. I've never really had an issue with standing waves and actually integrated one Martin Logan sub with some monitors several years ago with no problems. Since the CW IV does very well on it's own, I really need something with high output at lower frequencies. Hopefully I will get lucky and one is all I need.

Still thinking, but this is the way I'm leaning.



Oz

Get two subwoofers, one will drive you nuts. If you are using a larger driver in a properly designed sealed enclosure servo is not necessary. The larger the driver the lower the distortion because the larger driver does not have to move as far staying within the linear range of its suspension. This assumes drivers of equal quality. In a two subwoofer system 12" is OK, 15" is ideal as is 4 12" drivers. What you do need is a powerful amp with DSP control. Check this out https://www.parts-express.com/dayton-audio-15-ultimax-subwoofer-and-cabinet-bundle--300-7097 Then you install one of these https://www.parts-express.com/dayton-audio-spa1200dsp-1200w-subwoofer-amplifier-with-dsp--300-8000 and there you have it. Two 15" DSP controlled subwoofers for $2000.00. Spend just a little more and get the 2400 watt amp. These kits go together beautifully. If you are not good at putting on a finish you can cover them in black carpet or take them to a cabinet refinisher who can lacquer them black. Parts Express sells black carpet with an adhesive back.  This is a very serious subwoofer. The driver is first class and significantly better than the ones you mentioned. The amp is no joke either although I would save a little more and go for the 2400 watt version.
The future belongs to the brave so, get out that Gorilla glue and have at it:)

Out of stock for two months......  :-{