Keep in mind that unless you use the line level inputs (which you won't be doing), the subs from Rythmik do not have a very extended high frequency response. This is some kind of odd consequence of their servo system. Not such a big deal if you intend to crossover at 80 Hz or lower, but the high end roll off starts like at 70 Hz:
That doesn't give you much flexibility for integration, and you will want all the flexibility you can get since you will have no bass management. The Hsu subs are flat out to 200 Hz with any connectivity. And two subs can give you a much better response and a much better sound than one sub. Two subs can also greatly reduce localization that happens with a single sub.
Something else to think about that will help integration is sealing the ports on your main speakers if they are ported models. That will provide a much more predictable low end and make phase matching between speakers and subs much easier. It's a bit like applying a more controlled high-pass filter to your speakers. What speakers are you using?
