built-in Xover during active bi-amping.

If your crossover has separately adjustable crossover frequencies like the Bryston, you can probably get away with not removing the crossover although the net effect will not be as good as removing the crossovers, it will preserve the resale value. If not you need to remove the crossover.

If your crossover does have separately adjustable crossover frequencies, you can set the high frequencies to extend below the crossover point and the low frquencies to extend above the crossover point and let the passive crossover filter out the portion of the frequency spectrum above or below it's crossover point. The advantage is that the amplifier channels will no longer see the portion of the signal above or below their crossover points. The disadvantage is that you still have the passive crossover in there sorting out the frequencies around the crossover point.

To answer your question about the effect of leaving both in, think of it in these terms. Let's say you both have an active and a passive crossover which both attenuate the signal at 6 dB per octave (just to make up a number). The passive crossover was designed so that with it's 6 dB per octave the speakers would blend together and give you a flat frequency response. With the two crossovers in the picture you now have a 12 dB per octave rolloff. Looking at the situation from the woofers point of view, originally it was counting on the help of the midrange speaker to produce part of the frequencies below the crossover point. Since these frequencies are being rolled off more quickly that the designer intended, less of those frequencies are being produced by the midrange than required for flat response (assuming it was flat to begin with). The same thing applies in reverse to the upper end. The net result is a reduction of output around the crossover point.

Greg

As I mentioned, you will not see the full benefit nor will you lower your speaker's resale value unless you remove the crossover. This approach will always generate debate much like vertical biampping, since it uses ideas from both active and passive biamping. I would say there is a continuum of sound quality/cost which runs along these lines.

1) Standard setup, one channel per speaker
2) Vertical biamping, two channels per speaker Need more amps
3) Active/Vertical biamp approach (This one) Need more amps and a unusual crossover.
4) True active More amps, crossovers and speaker mods.

There are a few who stoutly maintain that passive biamping has no effect and will not improve your sound, since I've heard it improve sound, to the point where I bought another amplifier for that improvement, I disagree with them.

There are two main advantages to an active biamp over other forms of biamping.

1) "Getting passive filters out of the way" and thereby getting rid of all the nastiness like phase shifts and frequency response problems which passive crossovers are noted for.

2) Reducing the frequency range of the signal each amplifier channel is seeing. With a vertical biamp, the amplifier still sees the full frequency spectrum and attempts to reproduce it even though the portion of the spectrum which is outside of it's section of the speaker does not produce power since it is driving infinite resistance.

You will not get the first benefit with the approach I suggested, however you will get the benefits of reducing the frequency range each amplifier is seeing, and keep it from trying to reproduce the singal which will not be heard. Exactly how much of a benefit you will see is somewhat system dependent, since it relies on the original crossover.
Greg
BTW - The Bryston I was referring to is the 10B, it's up on Bryston's web site at www.bryston.ca.