I suspect the $4,000 price claimed is not real- likely seen by atmasphere somewhere on the resale market because the product is long discontinued. That price is not representative of what it really sold for back when it was made or what current drivers sell for that replaced it from TAD. I know the distributor in the US.
TAD DOES make expensive drivers, they are Japanese (Pioneer) but are used mostly in their own speakers or OEM in horn loaded systems. Very good drivers for sure. TAD was a good low distortion driver used regularly by George Ausperger in his large all horn designs which were quite popular in recording studios years ago. Today, TAD is still used by folks that build these all horn loaded systems that echo those Asperger designs. I hate the way they sound- high distortion and just loud. Lower fidelity by today's standards. They are common for hip hop where SPL matters on client playback. Mixing still done on lower distortion direct radiators (we sold TImbaland ATC 100s for this exact purpose).
Many studios still have these TAD or JBL loaded horn systems in the wall- they do look cool and are part of the studio "vibe". They were always used for the band to hear playback at higher SPL in the best possible fidelity (which is very low compared to modern hi fi or studio speakers). SPL = excitement. The trend now in studios is direct radiators, such as ATC, that favor low distortion so you can mix faster, worry less about translation and hear more details.
It is interesting that the TAD 1602 drivers had very little test data or info available. It does say the 1602"S" was a special version of it with shorter voice coil for lower distortion. You would have read about that same short coil idea in the white paper from Billy Woodman I posted. All the ATC studio and hi fi drivers use short coil long gap topologies for reduced harmonic distortion.
This is precisely what I am saying that efficiency is but one part of the picture as today short coil, venting, narrow gaps with precise coil fitment, flat wire coils, the right former material, etc is way more important when amps are relatively cheap. These types of drivers are impossible to build by machine. Hand made drivers is the way to get it done and everyone in the industry used to build their own: KEF, B+W, JBL, on and on. Now, hardly anyone does this anymore as it's way too expensive, difficult to train people and environmentally challenging. A small company cannot afford to build their own. ATC is the last of the breed in the UK. Everyone else has left and gone to China.
Brad