beryllium vs diamond

smodtactical, 3.7s are the cherry in the line cost considered and you will usually be able to hear them at a dealer. 
Carbon, beryllium, diamond, silk, paper mache, whoopee. Power handling is the big issue for domes. But you are still left with a point source radiator.  Using them in multiples has never worked well and probably never will. So, if all you are looking for is a midget sound stage they can be wonderful and I am not so sure the material makes an overwhelming difference. It is more important to find a speaker you like the sound of and will support the volume levels you aspire too and the best Tweeters will do this. 

I’ve heard enough ESLs and big membrane speakers to know what they sound like, and they all basically sound the same. They present a massive wall of hyper-detailed sound with no sense of space

So full of it, you have no idea, you just described horns.

Over many years of audio, ESL’s have given me the best imaging in both side to side, outside the speakers, and in depth perception. No dynamic I have heard even Wilson Alexia MkII has ever equaled them.

But they wouldn't hold up too long being fed a steady diet of, Daft Punk, Tool, and DCD

As for your band/music taste, it explains now so much why you are the way you are.  

Manners, Georgehifi.....it maketh the the man. Fine to disagree, less so to berate and belittle....

Hey Kosst_amojan the more a speaker disperses the worse it's image is going to be in any typical room. In open air they will image fine. I think your imagination may have the best of you. The membrane of an ESL represents a segment of the sound wave of a performance at a distance and it hearkens back to the stage in a way no other speaker can because it represents a segment of that sphere perfectly as there is no crossover. The ESL mimics the performance while the point source tries to mimic the performers resulting in a miniature sound stage and contracted depth as well as much more distortion and much poorer phase characteristics due to the crossovers and the nature of the drivers. ESLs can actually produce fabulous bass but you have to put them in an infinite baffle to make them work well which is totally and completely impractical for any remotely reasonable person so, the compromise is the subwoofer.
Because I am sure your hearing is just fine I know for an absolute fact that you have not heard an appropriately set up ESL infinite Line source system and they are not easy to find. Should you ever be in the Boston area I would be more than happy to demonstrate.
Now for those of you that want the ultimate point source speaker that will go loud enough to bust your eardrums you get four Accuton C168-6-490 midrange drivers and two C30-6-024 tweeters for a total price of $5000.00 Make a 2" thick plate 10" X 20" out of MDF and mount the midrange drivers on either side of the tweeter and as tight as you can to the tweeter. Make a simple 6 dB/Oct crossover at 3000 Hz. Cross to two subs at 125 Hz 24 dB/Oct. Lacquer plates to taste. I like Rasta. Don't forget to bull nose the edges. You might have to pad the MRs just a little. Hang the speakers from chains 1' in front of your wall right over the subs. The sensitivity will be about 93 dB 1 watt at 1 meter and the speaker should do 120 dB on just 100 watts! I would put acoustic foam behind the speakers. You would have to spend at least $30,000 on pre-assembled 
speakers to match these, maybe even more....much more. Life in the fast lane.