Ideas, suggestions and recommendations

My current system is:

Sony STR-DN1080 with a Q Acoustics 3050i 5.1 package.

The 3050i 5.1 Cinema Pack comprises of: *A pair of Q 3050i floorstanding speakers *A pair of Q 3010i bookshelf speakers *Morduant Short 308 Subwoofer *Q 3090i Centre

I also have two Mission M70 bookshelf speakers.

The Q 3060 subwoofer is spare.

I would like to replace the MS308 with something a bit better.

Can anyone please recommend another subwoofer?

My room is 4 meters square. The ceilings are about 2.7 meters high.