The TAD TD4003 has been discontinued according to a couple of supplier websites, including Pro Audio Design. I haven't heard Ale or Go To or Cogent or whatever other fabulously priced drivers I haven't even heard of, but I believe the TD4003 was a serious contender for best in the world. I know for some of you the TAD itself seems fabulously priced, so I will tell you that my context is that my kids are through college, I'm still working well past the expected retirement age because I like my job, and my personal vehicle I bought used 17 years ago for $7000. It's a choice that would be possible to a lot of people. Anyway, now for the DIYer who thinks large format beryllium compression drivers are a key it is Brush-Wellman or nothing.
I discovered this site several years ago looking for information about the LE8T to make TV speakers for a Sony CRT television. Somehow that segued to a pair of 4345's. One conclusion I came to early on was that ebay had changed the arithmetic of DIY and buying used made more sense than building; a bit of luck in the used market for a system was less costly--the whole was less than the some of the parts-- unless you aspired to world class speakers--and you can get the necessary components.
About that time Project May got started and I had a chance to hear some sounds that made me think it was actually possible to go for a world class DIY speaker. During a break in the Project May analysis we lifted down the upper 1500al woofer cabinet, removed the H9800 horn and driver and set a TD4000/H4003 in its place. The price of those components was for me a ridiculous sum of money to spend on hi fi, but it was so clearly superior to anything else I had heard that it became a matter of that or do something else for a hobby. Even then the horns were no longer available, and luckily I was some time afterward offered a pair of early trial pieces in the clone run at a wonderful price. Soon after I was warned that the TD4003 supply might become problematical and I plunked down for a pair.
In a way, I'm amazed they stayed on the market as long as they did. I was well into acquiring the bits for a wannabe M9500, so I kept to the 1400nd woofers and never got into any of the 1500al buys. I regret this now, although I think the 1400nd does not give away a great deal to the 1500al; still it is certainly a further development of the JBL woofer.
I'm still working on the exact arrangement for deploying these elements, but I have been so enjoying these speakers that it makes me a bit sad to think that none of the key pieces is any longer available. The DIY impulse is from one point of view iconoclastic and exclusive, but from another I have always felt it is framed by a democratic push.
I had heard some time ago that the TAD professional pieces were in short supply because TAD was doing better devoting its beryllium capacities to very expensive home speakers. JBL has made it clear it is not interested in selling components. Now I've read recently, perhaps here, that other manufacturers--Tannoy, Focal-- are following suit. I know there are sound technical and business reasons for making that choice, but I just don't like it that the driver manufacturers are corralling their best goods behind expensive veneers, beyond the reach of the working guy with a table saw. It's symptomatic of a larger arc that has been shaping up for a while and may very soon be anchored very firmly on its objective. But that is to go off topic and violate forum rules.