Hi,
It would be interesting to know which amps you're using on the B&Ws, they're good speakers but they do need plenty of power.
Also i can't remember if they use a port, speaker placement might affect your bass.
Apologies, just reread your original post, can you get a demo of another 4BSST2 to try a bridged pair?
That could well give the bass extra headroom and impact.
Depends a bit how that nominal 8 Ohm impedance matches with reality, another possibility with three ways is to get the extra 4B plus a pair of external crossovers as the low crossover point is at around half power.
Ideal for the 803s would be a pair of 7 Bs.
Tim