I would surmise that if you used a 60 Hz crossover point, you would remove stress from the very nice 803s, improve their lower midrange, and allow your amplifier a bit more headroom, too. The M&K sub will introduce the lower octave that you wouldn't otherwise have. I wouldn't run the B&Ws full range because you'll be duplicating the 40 Hz-80 Hz region, and the sound will be thick. Running fronts full range with a sub defeats one of the main reasons for using a sub, which is removing bass from a speaker that was probably designed with a driver to be used from 40 Hz to 500 Hz or higher, which is a compromise in the bass region.
Be wary of rabid purists who feel a subwoofer should never be used with B&W speakers. 99% of all speakers benefit from having a subwoofer in the system. Some B&W owners think bass is Pure Evil, even though B&W makes subwoofers.
To answer your question...it's a match, and a nice one.
