Interesting that you bring up DEQX. I tried a DEQX pre-amp (on loan from a local dealer) in my system before I went down the subwoofer route, and it didn't require replacing the crossovers. (A member on this forum highly recommended their products.) The DEQX is a fascinating (though expensive) product. In the end I chose a sub solution, because it couldn't solve my bass issues, and I didn't have upper octave complaints, but the DEQX might very well work interesting results with the B&Ws.
For equalisation, DEQX are excellent, but still subject to limitations.
With the higher end DEQX products you can use the individual outputs and go fully DSP active, and that's when they become very powerful tools.
Driving speakers such as B&W's, that would provide the user with control over the crossover frequencies and filter slopes, as well as time alignment between drive units, which then also makes it possible to adjust beam tilt for phase and time alignment at the listening position - including compensation for the height of the sofa etc.
B&W chose a slightly odd crossover design for the mid-treble with their 800Dx series. The tweeter should be wired out of phase but instead it's moved forward in the array by an amount that's equal to 180 degrees. This achieves phase coherence at the crossover frequency but it's a different story as you move up or down from there, which is why there's a slight measurable dip in the response at 6-7kHz and 2kHz. It also makes vertical axis response a little more uneven. The slight trade-off was to ensure better transient response and 'absolute' phase.
With a DEQX though, it would be possible to completely eliminate the trade-offs, which also includes time alignment and off-axis / power response. (I would probably move the mid-tweeter crossover down to about 2.5kHz)
DEQX is obviously expensive but I think we'll start to see much cheaper options available in the future.