I have tested quite a few brands of subs in my 10K cubic foot room. Some are simply terrible in terms of muddy concocted sound. The worst one (for my space) was the Paradym monster 15 inch sub with way too much power; although I can say that it is a very sturdy well built piece of gear. Nothing would tune it to the room, about all it did was rattle the windows; it was not musical at all. The next worst speakers I used were a group of SVS speakers, again pure mud and phasing them was nearly impossible. A few others were tested with limited success and then I brought in Bryston/Axiom subs and how refreshing! Absolutely harmonious easy listening natural bass which blends perfectly with the main speakers. I currently have 4 subs; the room is filled with fabulous full sound, no muddy issues at all and they are a snap to phase with the fully variable phasing controls. I set them up initially and rarely touch anything. I found the most perfect musical bass yet. I did not concentrate on size, that is all large or all small. I bought the size which fit the specific area for location in the room. Two are set up at the 90 and 270 degree positions and another is at the rear 180. The 270 degree spot is one where I ended up stacking a single 12 sub on top of a dual 12 inch driver sub; the stack is around 6 feet tall and fills the corner perfectly. Up front are a pair of Bryston Model T signature triple 8 inch woofer main speakers and no sub is needed in the front. I cross the subs at 150 hz so that I get all of the really nice upper range bass with the very low mids, that was key to the set up. I have the front mains crossed at around 80 hz so that they handle down to the mid bass and don't do the heavy lifting. My larger subs will handle very low range bass beautifully.
I am really pleased with the set up; the speakers were all built to order for me in real wood veneers with pleasing satin natural finishes.