First Order Crossovers: Pros and Cons

Cdc: forget the perfect piston. It'd be nice to have -- but let's just dream for now:)
An 8" driver has the advantage of being able to cope with mid & low-mid frequencies w/out extreme excursion. A smaller driver would be straining. OTOH, it WILL beam, as you hint, higher up. That'll narrow the sweet spot but, on the brighter side, IF the dispersion changes in a reasonably controlled manner, you reduce reflections... (at least that's s/thing). If the full-range can take it elegantly, I would try cutting in the tweet, 1st order, higher around 8kHz.
It's a pain to align the drivers -- but once it's done, it sounds good.
Cheers

Greg, I like the concept. The equation I use is 1,100/ freq = wavelength (ft). So a 2" driver would start to beam at 8K. How do you align the drivers? If you use a 1" tweeter I would think you get a flare if you drop it in at under 13K.
My concept is to use a 4" Tang-Band or new WR125S from Creative Sounds. Both roll off over 10K so you use the natural rolloff of the driver and add a tweeter with a capacitor to cut frequency below 10K. I'm sure my inexperience is showing here but this is what I'd like to do. Minimal x-over and F-R to 20k. Beaming would be a minor issue.
On your 8K concept there is Hammer Dynamics 12" but I think the main driver has a whizzer which I am against using. Here are Other single drivers
Have fun. Let me know what you think.

Diaural v. Acoustic-Reality what is the differance? and how do they differ from other series crossovers?

Driver aligment isn;t easy to do well (wavelengths are so short up there). A Supravox 215 fron mounted on the baffle and a tweet back mounted with a makeshift waveguide, blended quite well at ~8-9kHz. All open baffle.
bastanissells an open baffle kit called "prometheus airforce" that's reputedly very good (someone does sell import it in the US).
Do check the Tangbang's excursion capabilities -- I think you'd need to cross the Tang to a woof quite high up to avoid hitting xmax too soon. Also run a quick test to see if a small contour wouldn't be indicated (quite a few people at diyaudio have worked with the Tang).

As to the Hammer, I haven't heard it but given the many accolades (esp with a fostex supertweet) it may be an option. Good discussion at melhuish's site (super 12.
Cheers

Skrivis - Srajan Ebaen has recently written about the "Doppler Effect" in his quasi review of the Zu Cable Druid speaker. The following is an excerpt in which he mentions the Doppler Effect.

"Conceptually, single-driver loudspeakers (this one's technically a 1.5-way) are phase and time coherent though the Doppler effect could be cited when you consider how the high-frequency whizzer cone rides atop the woofer. The day-to-day observable Doppler effect occurs with police or fire sirens. They sound higher pitched as they approach (wavelengths shorten), then successively lower as they pass us and recede into the distance. Theoretically, each time the Druids' woofers move forward, they modulate the tweeter response. Once you do the math and consider the average stroke of this 10" driver -- to calculate possible tweeter response deviations in terms of how woofer distance traveled equates to wave length -- it seems more of a conceptual than audible problem. Still, it's only fair to mention in this context and avoid painting a picture of theoretical perfection. Clearly, if the single-driver ideal were the one perfect solution, nobody would bother with multi-driver designs. The market place rather demolishes any such notions in one brief instance. As usual, it's about priorities. What type of compromises are acceptable to facilitate certain concrete gains that matter more to you than that which is sacrificed?"

Here is the link: http://www.6moons.com/audioreviews/zu/druid.html