I currently have SVS, Rythmik and REL subs in my system. Of the three, Rythmik has the best sound followed by the SVS. The Rythmik has a little less output power than the SVS and the REL. The REL was the toughest to integrate due to the lack of a variable phase control. I also had to make a very minor modification to the REL so it would be compatible with the other two
I wondered if the Rel was as strong without the high level inputs.
IMO, I found using the line level inputs on the REL improved its sound. It was cleaner, i.e. less distortion.