Seas Excel Diamond tweeter or Plasma?

I think the idea of shopping for several best components based on cost and assembling them into a working speaker is quite a challenge. 

You may be disappointed, as the best tweeters in the world sound like nothing at all.  They  vanish.  They are boring. They call no attention to themselves.

You should do your own listening, but I've heard plasma tweets and remained unimpressed that they were significantly better than other top of the line offerings from Scanspeak, Seas or Mundorf. The Mundorf and Beyma AMT's are also among the best available, with the Beyma horn loaded AMT having some interesting dispersion benefits.  Dispersion matters just as much as specs.  There's no best, there's a best for a specific application and intention.

Also, unless you will build a Magico-like listening room with massive bass traps I would no longer try to build a monolithic speaker with a giant bass.  Using a separate sub or separately powered bass section with PEQ in the design is the way to go.

Thanks for the post. I actually already own my designer’s former top line speaker (Tetra 606) that has the Mundorf dipole AMT in a rear horn configuration. For sure they disappear and create exceptional “sound”. We are hoping to make something that is superior. My guy wants to make a 4 way with a smaller woofer on top. His plan is to create a sound similar to a duel concentric point source. He is a master with crossovers. Interesting that we have decided on an enclosure shaped like a baby grand piano. Slanted back to time align with beveled edges. He is confident that he can make it sing.
Another friend involved in the design also is against plasma. 

You'd better check with Kenjit before building anything.