Are first order crossovers best?

Bombaywalla...The link you provided looks interesting, and when I have time to spend I will study it. However, I must dispute your suggestion that the phase difference between drivers in higher order X/O does not remain constant at frequencies away from X/O. A second order X/O is basically a series connection of two first order X/O, and what works the first time does also the second time. A Crown professional electronic X/O that I keep around for experiments is 18 dB/oct, and has a comprehensive manual that shows graphs for both amplitude and phase response for various frequency settings. 270 degree phase difference exists at all frequencies (as I would expect).

My main speakers are Magnepanar MG1.6, and the high pass filter is first order. The low pass is second order, and the break frequencies are significantly different. Of course the Maggie "driver" is quite different from the common cone driver, but it all goes to show that a X/O network can be taylored to the characteristics of the drivers used.

I also use an electronic X/O for subwoofer/main, and it is fourth order. In prior systems, where the main speakers were biamped I have had excellent results with fourth order.

Unsound...We don't separate them. The full range advocates don't like to admit that their drivers break up, and emit different frequencies from different parts of the cone, and this constitutes a mechanical filter. If you think that design of an electrical filter is difficult imagine the work it takes to get a mechanical filter right!

Unsound...I had forgotten the Walsh driver...but, I think that even that had a mechanical X/O of sorts. The top part of the cone for a couple of inches (near the voice coil) was made of metal, while the rest of the cone was some other material.

Yes single drivers do have a mechanical filter being the driver itself.But there's a bit more to it than that.I think Wayne Parham explains it very well here Electrical filters and Acoustic Filters.Also when it's right ..it's pretty darn good.Martin King and Bob Brines have found a method that works well.Though single drivers..especially mass loaded transmission line models won't play at rock concert levels.What they do within their usable SPLs and frequency range is terrific.One of attributes of their design is minimal impedance and phase angle shifts.The difference can be heard if one were to only take time to listen.The distortion levels are the lowest I've heard in my home and anywhere else for that matter.My Magnepans didn't have the speed of these single drivers with 50 wpc,even with a 600 wpc channel amplifier on Maggies.Bombaywalla
thanks for the links ..this is good stuff!

Yeah I love my Driver cone breakup and distortion, too bad it still sounds best! Crossoverless Zu's see system. Not a single full range however, it uses 2 :)