Thanks for the IM Mr. V.A., very thoughtful. AV123 has a viable contender. May I suggest the Reference 3 http://www.av123.com/products_product.php?section=speakers&product=38.1
Unfortunately, it has 4th-order x-overs and some odd super tweeter for sounds only heard by bats.
Color me skeptical. 
Its called a magnetostat tweeter. They sell it separatly as well to add on to any speaker system.
http://www.av123.com/products_product.php?section=speakers&product=74.1 Please allow me the latitude to reprint a post I made in a thread on the AV123 forum regarding this tweeter.
"I do not own the ERT or any other 'supertweeter'. I am contemplating the purchase of one though since I do own a single driver speaker that MIGHT benefit.
Its essential to understand some of the mechanics of music to grasp what a supertweeter might do. Most instruments produce complex sounds. By complex I mean that in addition to the 'fundamental' frequency which is predominant, there are also overtones or harmonic frequencies produced that allow us to make the distinction between a french horn or a trumpet, a flute or pan pipes, a bass clarinet or bassoon. These overtones represent a small percentage of the overall output of the sound of a particular instrument yet allow us enough clues as to what is actually being played. By the same token, frequencies above 20,000hz represent a small percentage of the musical spectrum. There are some instruments that do have a significant portion of their content above 20khz.
http://www.cco.caltech.edu/~boyk/spectra/spectra.htm This page also cites the paper by Oohashi and the observed effects of reproduction above 20khz.
I am a baker by profession. If I forget to put the vanilla in a recipe, usually less than 1% by weight, the flavor, to me, is noticeably 'flat'. Then again there are some people who think instant coffee tastes the same as freshly ground gourmet beans. As usual YMMV.
Just my .02 cents, + or - .02 cents margin of error
BTW, I prefer Sumatran full city roast, espresso grind, brewed in a Bodum Mini-eSantos."
As to the 4th order crossover, I find that the proof is in the pudding. If its implimented crappy, it will sound crappy. If the designer knows what they're doing, you wont be able to tell weather its 1st order, 4th order or mail order, it will just sound 'right'.