The gear you already have will make the heart of a very good HT sound system. You will need to purchase speakers for the center and rear channels, plus a receiver or a preamp/processor to handle the digital surround sound, and of course a TV set.

You already have 3 Bryston 4B amplifiers that can provide 6 channels of high powered and high quality amplification. Why are you considering either the Denon or Onkyo receivers that certainly will be less potent than the Brystons? Why not buy a less expensive HT receiver that has preamp out jacks and connect it to your Bryston amps. You would be using the receiver only as a preamp and digital processor. If you want to spend the extra money, you might consider a separate preamp/processor, but they cost considerably more than a receiver.

As far as speakers go, the choice in center channel speaker is more critical than the rear channels. The center channel in movies has most if not all of the dialog and much of the sound effects too. The center channel speaker does not need to handle low frequencies as well as your main speakers, and many people successfully use center speakers that have the low frequencies cut off.

If you plan to use your B&W 801s as the front speakers for your HT system, listen to various center channel choices along with them. Bring the 801s to the dealer, or bring the center channels home. The ability to smoothly blend sounds from side to center to side is important. Often people achieve this by choosing a center speaker that has similar or identical midrange drivers and tweeters as in their front left and right speakers. Auditions of center speakers working with your 801s should make it clear what works well.

After you find a suitable center speaker, it should be much easier to find the rear speakers. Like the center speaker, the low frequencies do not need to be reproduced by the surround channels.

Good luck searching.