I have owned or listened carefully to subs from SVS, Polk, Klipsch, REL. I currently have subs from B&W and Monitor Audio. For two channel listening I prefer high level input over line level, so take that for what it’s worth.
I prefer the sound of the REL subs I’ve listened to (S/812 and T/5x) which I would characterize as being “musical” with “organic” but “fast” sounding bass over the SVS (SB-1000 and SB-4000) which sounded “clean” and slightly “sterile” to me.
I looked carefully at Rythmik subs when I was shopping, and nearly bought one because I like sealed cabinets, need high level and line level inputs, and they have a solid reputation, but I ended up going in a different direction for aesthetic reasons. I am sure that would be a solid choice.
No matter what direction you go, good bass is expensive, and I have no problem spending as much or more on the subwoofer as on a pair of speakers. If you elect to buy two subwoofers as many have suggested here, that won’t be hard.