Thanks all for your suggestions. I have KEF Refernce 2000 Cis in the rear at the moment with 3x 6.5" drivers in each. While this is isn't quite full range they handle a lot more bass than your average surround sound speaker. I'm pretty sure that the crossover is set to 75Hz in MConfig so anything below 75Hz is routed through to the sub. I know that with digital speakers bass handling is more sophisticated so, now I've thought about it for a few seconds your proposition makes perfect sense.

I thought that one of the disadvantages of the 8000 is that the 7200 sounds so good, I'm not sure whether I can create enough free space for the side firing Woofers.

I'm surprised that no one has suggested an 861. Apart from bragging rights would it offer anything?