Do these speakers exist?

Why are you even looking at 90db with an SET? Also, most amps don't have 6ohm taps??? Very few will be 4ohms and above 90db. You can not expect any type of performance out of a 20 watt amp with 88db. Stay away from thiels. They are amazing speakers but very hard on an amp.
Finally, a first order crossover put into this mix makes it tougher yet. If you can bend on the crossover, try Coincident Speakers. You will get 94db to 102db with a very flat 8ohms or higher and friendly to as low as 3 watt amps.
The other route you may want to look at is brand new Klipshorns. You will get flat to 25-30hz, over 100db, 3 way, 8ohms...

If you think 90dB speakers can't work well with higher power, 845 and up SETs, your experience isn't broad enough.

What you note about Thiels is apparantly NOT true of the new 3.7. They are supposed to be a much easier load. I don't know for sure.

I had always thought 1st order xovers were the EASIEST load for an amp. They are the simplest crossover - fewest parts - least loss of energy. I have read this many times, but perhaps it is wrong.

No ribbon or horns well hope you have listened to the new transducers but if you dont like nothing wrong with that. Have you considered a PHY 12in coaxil in a OB? Very good with SET amps. You could DIY a pair or have probuilt. I do custom work and would suggest a accuton c13-6 tweeter, accuton c90-t6 mid.Aura 12in woofers with built in 500watt power amps. system would be 6ohms 92db 1 watt and since self powered your SET wouldnt be clipping on deep bass notes.Depending on cabinet finish retail shipped arround $5000 I could offer a 90db version that wouldnt need a bass amplifier for $4250.Just a thought dont mean to give you a hard sell just thought you might like the option of a custom built system.JK-K.C.S.

Silverline is 1st order? That surprises me.

Here's a thought: the floorstanders from Reference 3A. They are not as strictly time and phase aligned as GMA or Thiel or Vandersteen, but otherwise I think they meet the requirements.

Hmmmmm... in my opinion, it makes more sense to choose speakers first and then get the amplifier to drive them. You may not be able to find a speaker that meets all your requirements including amplifier compatibility, but I'm pretty sure you can find an amp that will work with the speakers you want.

For instance, there are quite a few 845-based SET amps that will drive 4 ohm speakers no problem. Shoot me an e-mail if you'd like a couple of suggestions. That would put the Green Mountain Audio Calypso back on the table, which I think would be an excellent choice given your priorities.

Duke