For the best of both worlds I would keep your current setup for music and buy an av amp with multi channel pre outs. That way the av amp would drive the centre and rears, and your current setup would drive the front left/right speakers. All you need to do is connect the front left/right pre outs to one of the inputs on your pre-amp using a phono cable, and connect the centre and rear speakers to the av amp.
To set it up initially you set the volume on your pre-amp to say 12 o'clock (or whatever is easy to remember) and then the av amp will set up the rest of the speakers to match during the auto setup. If you want to watch a film you would need both amps on, set your pre-amp to 12 o'clock and then you can just use the remote control for the av amp to adjust the volume as normal.
For stereo duty you would only need your current set up on.
You wouldn't need to spend a fortune either, £400 will buy you an amazing bang for your buck these days. Look at Sony and Denon (maybe others...) as they tend to come with multi channel pre outs. Or even last years models may be heavily discounted.
Try and get the same make/series of speakers all round if you can, but especially for the fronts and centre.Anything from the RS series will be tonally matched. For the sub you can go for whatever manufacturer you wish. Lots of options and prices.