Small room or big room?
Movies or music?
Budget?
Preference for sealed box or ported?
Budget?
I recommend SVS subwoofers, I've had a few over the years. From my experience for music, in small to medium room go sealed box. For medium to larger rooms, for home cinema go ported. I would recommend the SVS SB or PC Plus subwoofers, or you want a high end sub, the SB or PC Ultra subs. There's the 16" Ultra that looks amazing.
Personally I would recommend you don't spend less than £400. Something like SVS 12" sealed box, or BK XLS400. Don't waste you time on a cheap £150 sub as it just is boomy, distorted sound that can't play low, or loud without high distortion.
