Regardless of Millercarbon's snarky condescension (per usual), RELs are NOT inherently "lumpy" unless set up badly, and a single one can work. Not everybody has room for a Swarm of four subs and many can get along with one well placed REL or another well designed brand. And there is a big difference between high level power amp driven signal and a single ended preamp signal as ideally the sub should sound like the amp, and the high level REL style works swimmingly. For reasons noted by atmasphere and others, using two subs (I use 2 RELs) can eliminate standing wave issues and get a room in tune easily, but if you can't afford 2 subs, simply get one and add another later if you feel the need.