Do you biamp your B&Ws or other comparable brands?

Absolutely agree with Czarivey. The best case scenario for passive bi-amping is 2 identical amps in a vertical configuration, however it is better to just have a single larger amp.

Center channel TBD

Isn't the obvious choice a B&W HTM2 Diamond?

Adding a center speaker, you will then be using all 5 channels of the A51 for HT. Are you then going to disconnect everything and reconnect for bi-amp to do stereo only listening?

When you drive the 803D with a single channel, the majority of the power is being used by the LF input, with a small amount of the power being used by the HF input. So, to bi-amp, you are just adding more potential of power for the HF input that is just not necessary. I agree that more power can sound better at low volumes, however, that extra potential of power needs to be available for the LF input, not just extra power on the HF input.

Something that you may not be aware of is that the A51 will have more output capability when only using 2 channels as opposed to using all 5 channels. Check the measurements here for 2 channel vs 5 channel.

Is bypassing the b&w crossover easy?

No, as this will require serious disassembly of the speaker since the crossover of the 803D consists of 3 separate boards in different locations inside the speaker.

If I do not end up biamping, should I at least biwire?

This has been debated extensively on this forum with various opinions. IME, I prefer single wire with matching jumpers to replace the factory straps between the LF and HF connections, although I believe the 803D comes with a much better jumper than the typical gold plated brass factory strap. If you bi-wire, I believe most agree that it is best to use 2 separate runs of identical cable.

Regarding centers, I may get a different center as my cabinet isn't tall enough to house the tweeter for the matching center.

Understand, but it will definitely need to be a B&W.

This may be a dumb question, but since all five inputs to the amp are connected to the preamp, do I need to do anything (setting, etc) so the amp knows only two channels of audio are being sent?

See page 46 in the preamp manual. Just press the "stereo button" on the remote is all you need to do. Then only the front L & R will play, and no signal will be sent to center and rears.

Does it matter which channels I use for fronts, center and rears.

Probably not, however, looking at the back of the A51, color coding indicates that,

Ch1 - R front
Ch2 - L front
Ch3 - center
Ch4 and Ch5 - rear, but no indication for R and L, so just make sure you are consistent with connections.

You're welcome, and glad to help.