Paul,
A crossover for your system may be a little bit of overkill.
The typical use of a crossover is to create "discreet" channels for each speaker (sub, mids, tweeter) so that each one can be separately amplified. It also allows for you to select VERY specific frequencies to send to each speaker. The frequencies that are allowed through to the speaker are often refered to as the "passband"....(the band of freqs the speakers will play)
The reason I see it as excessive for you is that you are running coaxial speakers off a 4-channel amp, right? That means that the tweeters are running off passive crossovers (caps and coils) built into the back of the speaker....you send in a full-range signal, and the speaker splits it between the mid and the tweet with it's built in crossover. One of the benefits of using a 3-way crossover is that you can have dedicated channels for the tweeters and power them directly off the amp....
The problem is that it takes a LOT more amplifier channels to really take advantage of a 3-way crossover....and typically you don't use crossovers like that unless you also are running component speakers (separate mids and tweets)
Let me give you a very simple example of how a 3-way crossover might be set up....this was my K5 system:
(Channel 1) Front Driver Side tweeter - 200W
(Channel 2) Front Pass Side tweeter - 200W
(Channel 3) Front Driver Side Mid - 200W
(Channel 4) Front Pass Side Mid - 200W
(Channel 5) Left Subwoofer - 200W
(Channel 6) Right Subwoofer - 200W
(There are actually 2 more channels that run a second midbass driver on each side, but they use the same outputs as channels 3&4 above)
With a setup like that, I can now make VERY specific adjustments for the exact frequencies that I want EACH speaker to play....and I can also adjust the output level (loudness) of each speaker since they each have their own amplifier channel. You may notice that I am not even running rear-fill speakers with that system, but even without them, I needed 6 channels of amplification to accomplish what's in that list (in reality I used 8 channels, to run the extra mids....a total of FOUR 2-channel amps!!!
With the system you have, the only thing you'd gain is the ability to select crossover freqencies more carefully...and to roll off the bass that you don't want to end up in your smaller satellite speakers. Those "bass blockers" would do the same thing in a passive component and would probably save you a lot of hassles.
IMHO, the 3-way crossover is not going to get you much unless you decide to swap out your coaxial speakers for component speakers, and you're willing to add more amplifiers to give yourself more amplifier channels to work with...
