| It is a good question that I also asked when I saw this speaker. Afterall, everything being equal using the same materials for the drivers is more optimal from a coloration standpoint. My guess is cost. They already had splurged for that hellishly expensive midrange and they want to keep the speaker priced in a range with the Wilson Watt/Puppies of the world. Add the diamond tweeters and now you are looking at $40,000+ speakers. This Supravox tweeter was perhaps the best they could find and still kept the price in the (more or less) real world.
Personally, if the diamond midrange is as good as they claim then I think it makes sense to push it as low as possible. Of course going too low and you run into distortion and dynamic limitations. I have found that speakers with a wide bandwidth midrange often sound quite good because the coherence through a critical range is good. My guess is that they can run this mid easily to 5Khz without issues. Perhaps even to 10Khz before the dispersion really starts to suffer.
I would not dismiss this speaker out of hand because it does look like a serious attempt to make a good sounding "conventional" speaker. I see nothing wrong with them touting that they give you more expensive parts for the money. It also throws a bit of mud in the face of other really expensive speakers when you see what the drivers cost (and that is retail prices which for sure the manufacturers don't pay), which I find kind of funny.
It reminds me of when Bobby Palkovic from Merlin tried to convince me that his $10,000 two-way speaker was justified in the price because it was so expensive for him to build. I calculated that there was only about $600 per speaker in materials...
Has anyone heard the Marten Design Coltrane Supreme to know what the diamond mid and diamond tweeter sound like? I have heard Lumen White speakers with Diamond tweeter and it wasn't bad at all with Ayon Reference 52B monoblocks. The Avalon Eidolon Diamond sounds boring with a Pass labs setup.