high-efficiency loudspeakers

PWK Khorns for decades. They have a deep sound stage, when properly voiced with good quality amps. Quality is key. And most of my listening is done slightly below 90dB.

In the last 60 years of this hobby I've heard systems that sound as good or only different, but none better. And none that delivered a "live in the next room" sound.

I have Martin Logan ESL's in another room, which are extremely transparent but sterile. I like the MLs, but they are not very involving. They provide a good reason to talk about and discuss amplifiers with other audiophile. Good material for helping selling amps in audio rags. Had Maggie's, Quads . . . same issue despite throwing tons of watts at them at any level. Odd, that I still like the MLs.

Hello all,
First, wtf, you're going to LOVE the Decware. I've been playing around with that amp since 2004. Done more mods on it than you can shake a stick at. Never felt a need to go the true DHT route. ( And I've heard a bunch of them ). It's just that good.
But of course you need the right speaker to mate with it. Let me throw my two cents in for the Decware HDT. Been using that also since 2004. That combo replaced a very high end system of Sonus Faber Guarnieri Homages/REL Stadium III subs driven by solid state electronics. Wouldn't dream of going back.
The HDT does all the things exceptionally well that single driver loudspeakers are noted for. Plus its WAF is very good. That combo gets plenty loud enough for me.
For the record I have no stake in Decware other than being a happy customer for a long time.
Cheers,                   Crazy Bill

Daedalus Audio speakers are very efficient and beautiful to behold too.

While there is no "best", I will say that Legacy Audio comes close..I've heard most of the above mentioned brands, and Legacy is the one that captures my heart and soul. A very efficient speaker not needing a lot of power to drive, but at the same time will handle a huge amount of clean power. Even the Legacy bookshelf speaker (Studio HD's) have a awesome sound, and have sound much larger than they actually are...they also score well on the wife factor, with their custom finishes she will find something that will fit her taste..the Rosewood finish did the trick for my better half.
www.legacyaudio.com

@bbarlow The Spatial Audio X2's look very promising indeed, and they could turn out to be real contenders.  I'll be watching the thread on A. Circle carefully.