In my system I am currently using a REL S5 SHO, a Rythmik F12SE and two SVS SB4000’s. My room is 10 x 14. When I first started using the REL, I used the speaker level input but have since changed to using the rca inputs. To me using the line level inputs there was slightly less distortion giving me a slightly cleaner sound. The Rythmik has the best sound of all three though it has a little less output power. The SVS has the most output power of the three, but the sound quality is not quite as good as the Rythmik but a little better that the REL. The sound quality differences between the three are noticeable but not substantial. If you are not using some type of external crossover the Rythmik and SVS also give you more tools to help with integration.
If multiple subs overwhelm a room that is just an integration error and can be easily fixed. The intent of multiple subs is not to create more bass, it is to balance out the bass through out the room. It is a type of acoustical treatment for the room as explained by MC and Big Greg.