£300 seems reasonable but I think you'd be better off with a B&K XLS200 for that money unless you can get a much better secondhand sub for the same money.
I may be out of turn here but I have checked out a few subs KEF's included and although they sound low they don't go low, even the manufacturers quoting lowest values of 30Hz ..... Mission are even worse ... and sorry but REL only report 25Hz as lowest on a similar sub, my floor standers can achieve 35Hz for crying out loud. Trouble is most sub manufacturers don't seem to want to tell you at what sound pressure (volume) levels they made their measurements.
Bass is supposed to be about filling the hole below the lowest your mains will achieve down to 20Hz and below (where you feel more than hear).
The B&K at least claims 17Hz at -6db (bit antisocial at that volume but hey the neighbours gotta go out sometime

)
I too am seeking a sub but size of amp and box aren't the only considerations, there needs to be some thought what function the sub is to fulfill and what the rest of your system is capable of - not to mention the size of the room.
Loud / Big or expensive does not necessarily mean good .....
Again sorry if anyone thinks I speak out of turn ....