I've always been told that unless you have complete control of subwoofer placement in your room you'd best be getting 2 to make sure that you can level match and not have directional bass. With that in mind, if I had the desire I'd be looking at Outlaw's LFM-2. Compact package, plenty of extension (in my book) and affordable enough so that I could run twins. Doesn't have continuous phase adjustment but has 0-180 switch which has been enough in my experience.