I am biamping my B&W HTM center channel speaker now with a single stereo Threshold s200 Stasis amp, but as you noted, that still uses the passive crossover built into the speaker. There is an external set of jumpers on the 5 way binding posts on the back of the B&W's that can be removed to break the internal connection between the woofer and tweeter.

I have gone the complete biamp route with a pair of Magnepan MG3a's a number of years back. The crossover for those is in an external box rather than being inside the speaker, so it can be entirely removed from the audio path if desired.

I used a NIKKO brand external electronic crossover in that application. It was hooked up between the preamp output and the power amp inputs. I used a pair of Threshold s200 Stasis amps with excellent results that improved on the sound delivered using the magnepan passive crossovers.

I still have that Nikko crossover sitting in it's box in my storage room. I don't have a use for it at the momenyt but it is far too nice of a piece to get rid of, and new high end electronic crossovers are rare and rather costly.