FWIW, Both my main and bed-room systems are actively tri-amplified and I've heard other successful and unsuccessful realizations. I was quite happy to leverage Siegfried Linkwitz's (as in Linkwitz-Riley) design work which just left the soldering to me.
>What are the best active crossovers?
The answer is most likely irrelevant to you because you don't yet have the background to recognize what you need.
Read _Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms_ by Floyd Toole for an idea of design targets and _The Loudspeaker Design Cookbook_ by Vance Dickson for some information on realizing those targets.
>I have a pair of Dunlavy SC-VI, which are four-way 6db slope crossover.
The cross-over network also compensates for driver resonances, applies baffle step compensation, and accommodates the rising response which goes with drivers with low inductance motors as they become acoustically large.
Some of John Dunlavy's cross-overs also adjusted driver Q which in turn affects what happens moving farther beyond the driver's pass-band, phase, and how the speaker sums within the driver's pass-band.
Krell's manual suggests that this can be accommodated, although that would mean paying some one for custom work and you still have topology issues to deal with.
Other generic active cross-overs lack any provision for that sort of thing and won't work well for you. The Pass XVR-1 isn't enough.
>I understand I would need to use Three Krell KBX in series to get 4-way 6db slope (besides 8 channels of amplification).
That won't duplicate your existing cross-over functions that have a lot to do with how your speakers sound.
Passive cross-overs are usually realized using band-pass filters on all but the woofer (low-pass only) and tweeter (high-pass) only. By cascading filters instead of using band-pass filters on the upper and lower midrange drivers you're going to change their phase relative to each other, how they sum, and what needs to be done to their relative levels to compensate. That changes your problem from copying to cross-over design which is a lot more complicated.
>Do you know of any high quality 4-way active crossover that can be configured to 6db slope (except for the 3x KBX option)?
The mini-DSP 2x4 will work for your purpose. Of course, using it will require measuring your passive cross-over transfer functions, and creating DSP filters to match.
That said, if you like your speakers you're better off not trying to "improve" things via active-cross overs unless you can purchase a unit specifically designed for your speakers.
If you want a DIY construction project (perhaps active) find a respected design and build it.
If you want to get into speaker and cross-over design as a hobby after reading both books I referenced, buy yourself some measurement software + hardware, perhaps some cross-over optimization software if you care to do passive speakers, and start with a 2-way.
