I had a Cambridge Soundworks DT3500 system, but the head unit failed.
Very, very good it was too, up 'til then.
Like you, the corner speakers were fixed in awkward places, so I bought a Creative something 5.1 system, and just replaced the head unit and unpowered sub with the new powered sub and cabled all the other existing speakers back to that - fortunately there was enough slack in all the cables to do so.
It isn't as good as the DT3500, though.
SD