I know Leif quite well from having owned Marten Coltranes before and spoken to him on many occasions. These new speakers look interesting. I saw the new Accuton sandwich bass drivers recently when I inspected the new Coltrane Tenor's in my Dealer's showroom. It looks like they're being used in the upper bass-mid-bass region in this application.

In regards to the two diamond tweeters you mentioned. Actually the larger diamond driver is a Diamond midrange. You mentioned he will be running 2 x diamond midrange & 2 x diamond tweeters. That would mean he is adding a rear-firing diamond midrange & tweeter, but the open cavity in the mock up suggests he might be using a transmission line mid/tweeter arrangement?

Following on from the above, Leif is now using the latest Cell-concept diamond midrange and tweeter which have lower distortion than the previous generation, better decay & are inherently time coherent in a flat front baffle as they share the same acoustic center. But i'm surprised Marten are not using the latest black-coated ceramic (BCC) drivers as per Avalon's new Tesseract, which by the way also uses the Cell-concept diamond midrange and tweeter & comes in $200k cheaper? I can only assume they couldn't reach an agreement to use these drivers