JP,

I have two ideas for you to consider. One would be to consider an active crossover (models from Rane, BSS, dBX, DEQX, Behringer, etc.) if you do not wish to develop your own crossover. Now this way will necessitate multiple amplification channels (for a three way crossover a six channel amp) and perhaps other considerations but it would do the job. This approach would be the science fair project in that you would need to work out the crossover for whatever choice of drivers that you might have. Plus some measurement capability would be needed to more easily deal with the crossover point and levels.

The second thought is to contact Rick Craig at Selah Audio (www.selahaudio.com) and ask him about his Linus 3 line array kit that uses the Dayton (Parts Express) RS drivers as the woofers. This kit is not shown on Rick's website but he does have a photo (linked) in his gallery. Rick uses, I believe, the RS150 woofer together with the Fountek Neo CD2.0 ribbon tweeters (that is a true ribbon driver) and he has worked out a passive crossover for this combo. I suspect that this parts kit may cost a little more than your budget ($2400 at last report) but it would be a significant step upward in performance vs. the other options you are considering.

Jim

Comments on your points first:

1. Yes on the 8 Fountek NeoCD2.0.
2. Essentially yes. With the Neo2.0 the crossover can be low enough so that trimming flanges would not be necessary.
3. You don't necessarily need to power taper. If you do, just a little.
4&5. Essentially yes but clearly a passive crossover would be best for less complexity (just a single set of stereo amps and less wiring). An active crossover would offer more flexibility if you like to tweak.

Now for your questions:

1. One enclosure is usually preferred but it usually partitioned with internal braces to strengthen the cabinet. You could thicken the cabinet walls as an alternative but braces are really superior in my mind to a cabinet that weighs twice as much.
2. Line arrays are more complex to design and build than other speakers so one would not go into them without either build a proven design or having the expertise to work through the details.
3. Selah Audio (and several other small custom builders) is a custom builder that builds speakers per requests from their customers. So you'll see far more variety in speaker designs and finishing options vs. a company that has a large factory assembly line operation to satisfy a specific product line for their dealers. Selah builds turnkey fully assembled systems and can supply parts kits for customers who wish to build their own enclosures. It is a lower overhead, lower cost, and more flexible way to do business versus high markup high end audio.

Jim

The RS180 is the best value of any 6.5"-7" driver I've tested (and I've tried about all of them). The P17 and PL17 are decent drivers but older technology.

http://www.ascendantaudio.com/poly_6.5_page.html