It's looking like HSU might have the best balance for my purpose.
Since you have a small room sealed might be your best option as a sealed sub would be less likely to excite room modes. Good reproduction of the last couple of octaves, especially the last octave is difficult. Unfortunately commercial subs play the numbers game.
So I'll give you as short a version as I can.
Loudspeakers are lousy acoustic couples to a room, and the bigger the room the worse they are. Now manufacturers practically never tell you the Q of the driver. This is important as total system Q can never be less than driver Qts. So any sub with a sloppy high Q suspension will never sound good.
In a sealed alignment you can get Q at driver Qts. However without Eq F3 is way above driver fs. The end result is that good sealed systems are expensive, requiring lots of power, and drivers that have high excursion. However this brute force approach, leads to distortion rising rapidly with decreasing frequency and dynamic compression under the best of circumstances.
So optimally a driver needs an acoustic transformer. That leaves ported boxes, pipes and horns. Of which by far the best options are pipes and horns. However they take up a lot of real estate and take a lot of experience to engineer. Unfortunately most commercial subs don't sound good as they go for lowest F3 whereas lack of ripple and lowest Q should be the design goal. That is why you don't want to plug a port on the HSU sub. In addition to lowering bass quality and power handling it also has port chuff if you do that.
So what of your choices?
The SVS seems tuned very nicely and is good value for money.
As is typical for these designs distortion goes off the clock as the driver decouples from the box. Distortion is minimal at box resonance where driver cone displacement is minimum.
The HSU is a pretty decent sub, and optimized for high output and roll off at 30 Hz. In your room not a bad thing.
The Rythmic is a servo sub, which is one way and a good way of equalizing a sealed box.
Frequency response is respectable.
As is typical for the breed, there is significant power compression which your room will mitigate.
Also distortion as anticipated is high.
For comparison here is what I think is optimal for the lower octave reproduction. Unfortunately, it requires a tolerant wife and a lot of experience to design it.
These are my full range dual lines.
I made these measurements as loud as I could stand it. Note the frequency response is smooth and roll of is indeed second order, as the pipes are damped just to the point where there is one peak of impedance. There is slight 5db ripple where the pipes transition. The measurements are taken just below 100 db. These are truly full range speakers.
It is a myth that pipes have high distortion. When calculating db to % THD, the maximum bass distortion at that power is 2% THD.
It is also a myth that pipes have poor impulse characteristics. While this shows a little over hang the result is superior.
At least for me, and I've been designing pipes for 60 years now, this is the most ideal bass loading I know. Unfortunately it is up against practicalities.
It should be noted that these pipes are doing this with standard audio drivers, not sub drivers. One speaker was used containing two 10" SEAS Excel metal coned drivers.
