what sub

Iam looking for a sub that will provide me deep fast bass i can use for 2.1 and 5.1..

my room is 15" x 14" do i go for a single sub or a pair ?

my kit is as follows:

restek challenger hif amp, yamaha dspe 800 processor, sony bluray, rotel rcd 991 cdp, sumsung 46" led tv,

mission 753 fronts, mission 75c center, b&w solid rears, b&w asw1000 sub.

I have looked at all sorts bk xxls400, mj acoustics pro 50, ref 1 mk2 , rel q200, strata5, b&w610xp ............... budget depends on 1 or 2.. im happy to buy a good used sub but up to about £650

any help and advice will all be taken on board.