If I understand right you have a 950, a 5 or 7 channel amp, and your subwoofer.

I think the best way to set it up would be to connect your B&W Matrix 805's directly to the amp, not through the subwoofer.

For the subwoofer, run a cable out of the sub out (LFE) on the 950 and get a Y adapter so that you can go into the L/R input in the sub.

Then you can set the front speakers to small on the 950 and it will send the low freq to the sub. You would probably only want to us the subs crossover if you had a receiver without bass management

As for the sub crossover, you can just turn it all the way up and let the 950 handle it.

If I misunderstood your question, sorry.