Great thread. Time to goose it!
Some anecdotal observations. I've been into audio awhile. Prior to the introduction of the B&W DM6, in 1976, there was not much attention paid to t&pa. Of course there were the Altec and JBL coaxials, as well as the Magnaplanar Tympani IDs, which I owned, the Quads, and Lowthers, which have been around forever. But they weren't touted as being linear phase. Then in 1973 the Dahlquist DQ10 appeared and it seems more and more designs since then are more cognizant of these considerations.
I'll never forget the first time I heard the DM6s. Until then I had no conception of sound stage. The DM6s changed all that. The DQ10s had that attribute as well. That is, one had a more realistic sense of the acoustic space the recording was made in. The Magnaplanars, to some extent, had that quality, but being dipoles, they had to be in an almost perfect acoustical setting before that sense of "acoustic disbelief' was evident. More often than not the room they were in influenced the sound to the extent that it became a 'wall of sound'. Not an altogether unpleasant experience mind you, just not a realistic sound stage.
The essence of the t&pa benefit is the realistic sense of sound staging. But, as its been stated in previous posts, its the implementation that determines the success of the design in this regard. In 2 or 3 way designs the interaction of the drivers and crossovers can trip up even the most careful designers. It's also been my observation that in all but concentric, or coincident designs where the tweeter resides either in the center of the cone or at the rear of the bass driver, the sweet spot is very narrow requiring 'head in a vice' seating. The reason for this has been explained very well in previous posts. The latest innovation to mitigate,to some extent, this effect is to use an open baffle, or dipole, in the midrange driver.
In single driver designs the compromise made is that dispersion suffers as frequency rises leading to a lessening of the sweet spot as well. This is not all bad as this lessens side and floor reflections of higher frequencies (its not a flaw, its a feature!). The other penalties of extended range drivers is linear frequency response, and fall off at the frequency extremes.
So what to do? In my case the journey has been influenced my exposure , as a younger man, to a couple of full range JBL drivers. As a sophomore in college a friend, who I worked with on the coffee house sound crew, purchased a couple of 4 1/2" full range JBL drivers intended for car use. He cut some holes in large cardboard boxes and filled them with dirty laundry. We were both amazed at the quality of sound that this dorm room setup made. Later, when I was in the sound reinforcement business, I heard a PA consisting of columns in which 5 10" fullrange JBL resided per side. These also impressed my with a coherency that was rare at the time.
A couple of years ago, when I started researching a new speaker purchase, I came across Audiocircles, as well as numerous other web sites devoted to music reproduction. It became evident to me that in order to successfully implement a multi-driver system with anything other than first order cross-overs, one needed to use outboard electronic xovers. The added expense and complexity of such a system all but ruled out this alternative for this frugal audiophile. The growing interest and technological innovation in extended range drivers brought back memories of those JBLs I'd had experience with. Thanks to DMason's insatiable curiosity, and seemingly unlimited budget, my 'leg work' was all but done for me as he explored the best of the best when it came to coaxial and full range drivers. As a result of his explorations, I settled on the Omega Aperiodic 8, to which I've added a Rocket UFW-12 subwoofer, as my main system.
To be sure, this is not a perfect solution. No design currently devised is. I can't back this up with scientific data, but experientially speaking, having listened to hundreds of speakers, it is very rare to have a conventional multi-driver system, with the drivers mounted vertically on a baffle, convey a stable, convincing, wide sound stage without spending over, at least,$3000 . To those of us who have made the leap to extended range drivers, with supplementation, we are unlikely to go back to multi-driver designs where a cross-over is used where the ear is most sensitive, ie 80-5000hz.
I realize I have made some gross generalizations regarding the use of time alignment and phase alignment interchangeably. They are indeed 2 different things. Also, there are unique examples of speakers that do not conform to the general broad statements I have made. Please refrain from rejecting the general thrust of my observations out of hand due to those exceptions.
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