You have one of two distinct paths to take depending on what you are willing to do to accomplish each. If your speakers have internal crossovers (they do if they function with one amp connected to the terminals), you really should not use an additional crossover. You could use a crossover to feed your speakers frquencies above say, 50 hz. Example would be something like the NHT X-2 (NOT the X-1, it has special EQ for NHT's own subs) to simply keep low frequencies from your speaker. This is common and not too much of a compromise if any at all. The other route is to go inside your speakers and bypass the passive crossover. I just looked up your speakers, I have to say I would not be going inside them myself. The company makes pretty much state of the art speakers, I think outboard x-over would sound different, not better than what they came with. Sort of like putting glass pack mufflers on a BMW M5. No performance gains, but it sounds different alright. If you must proceed with this, simply remove each driver and disconnect the wires from each terminal. Do the same for each terminal on the rear of the cabinet. Before you secure them all back in their positions, attach new wire to each driver and attach this same wire to it's corresponding terminal (where you removed the wires from the inside of the terminal). I say solder the wires to the terminals, you can solder to the driver tabs but be very careful not to get them too hot or you could damage them. To get the wire from terminal to mid range and tweeter you will have to make a small hole for each wire thru the internal baffle that provides these drivers their own air space away from he woofers. The woofers should be easy to wire from the lower terminals, take note of how they were wired by the MFR and wire in the same fashion. Most likey the positive goes to one pos tab of a woofer then to the same tab on the other woofer, unless they have push pins then it's easier. Do the negative wires the same way using the neg tabs. Be sure and apply some caulk around the holes for the mid/tweeter wires to maintain air/sound integrity. At this stage you simply need 3 decent amplifiers, 3 pairs of speaker cable, 3 pairs of interconnects. I say use XLR cables, they will sound better. Your amps need only be 120-150 watts each except for the woofers, they can use 200 or more. Best deals for the mid and tweeter on Audiogon are the 120 watt Parasound Halos or B&K Reference of similar wattage. Bryston 2BSST or 3BSST are probably best for this, but cost a lot more.
And now to burn the barn regarding crossovers- I have not read anyone describing the real differences between what seem to be the 'big three' quality active crossovers: Bryston B10 standard balanced, Marchand XM44 balanced, and the Rane AC22/23. The Bryston is $3K, and only has gain for mid/high, not the low end. You must rely on your pre amp for that. It has something very few x-over's offer: adjustable crossover slopes, although not as steep as the other two. You are paying for that and it also takes up some real estate inside the chassis. I have read where others point to that 'lack of crowded circuit board' as a weakness of the XM44 in direct comparison to the 10B. To me it is a little gimmicky for wht it brings to the table in non-studio use. Maybe that is why no others have it, nobody is asking for it? James Tanner will not say it but that feature has a negative effect on the signal. Compared to not having that feature anyway. It is also only a two way stereo x-over, you need a pair to go 3 way, $6K!!! The Marchand XM44's ($1,300) down side is the frequencies are fixed based on what you order- once you get it you cannot change frequencies without buying new modules at $100 each plus labor to remove/install new ones unless you are savvy. I do believe that the XM44's balanced frequency module is sonically suprerior and a bit more accurate than any available rotary/adjustable scheme thatmakes contact with several circuits of different value. Take a look at the Marchand modules on their site. Just compare the module to Bryston's rotary multi- frequency scheme. Now imagine if you had a module of the XM44's quality (and size, 2.5 by 5 inches) for every available frequency on the Bryston 10B standard's two frequency knobs. The 10B would become a MASSIVE component costing much, much more. The XM44 has gain for each channel and you can order slopes that are much steeper that the 10B, 48db per octave. The Rane is the most flexible, with gain for each channel, adjustable freqs for each channel, even adds delay if you need it. Rane is balanced only. You must order the Marchand as 2, 3 or 4 way, the Rane is variable 2 or 3 way but I think you need a pair to go 4 way. Rane $450, Marchand XM44 3-way balanced $1300, Bryston 10B $3000. A perfect crossover would combine the Bryston and Marchand- ditch the variable slopes, have gain on each channel with adjustable frquencies. The Rane has that although it does not have the premium parts, low noise or low distortion specs as the Bryston and Marchand. If you are a set-it-and- forget-it guy and you know what freqs and slopes you need, the Marchand is the answer. If you like to fiddle, tweak and expiriment the Bryston or Rane is the ticket, depending on budget. The Rane and it is really quiet and a solid performer if you just want to wade in slowly it is a good value and the most verastile should you change your system. The Rane has the option of being switched to either 2 or 3 way stereo in a single unit, the others do not. To my ear the XM44 and 10B are both dead quiet and I can't pick out which one has the more 'trasparent' sound or better dynamics. IMHO there is defintely not a $2,000 difference (for 2 way stereo XM44 as 10B is 2 way stereo also), that part is a no-brainer. So there you have it, a year of switching cables and long listening to maybe help you make the call. It was fun but of the 3 contenders, one is a permanent member now, bi-amping Magnepan 3.6Rs with Bryston 4BSSTs, guess who won? I suppose it's obvious I ordered the XM-44 with frequency modules and slopes for the Maggies. Besides it has that cool retro sci-fi look also...but I don't want this to be a plug for Mr. Marchand here. 3.6R owners, make the move and get that cheezy little box off the rear your speakers.