high-efficiency loudspeakers

You really want high end and ultimate efficiency?  The James B Lansing corner horns of the late 50's can't be beat.  The ones that have a chance for stereo placement in typical rooms are the C-34 "Hartsfields".....rear-loaded horns that do alright away from the corners (although corners are better).

Living Voice also makes less expensive speakers then the Vox Op their line of regular speakers are very good and an easy load for SET mine the Avatar 2's are 94db @6ohms nominal. I came from ANE's good but the living voice are better imo. also noted living voice tests/voices  all their speakers on Kondo gear so definatly good with tubes. I push mine with 300b love from Finale Audio / Triod Labs.

My main system Wilsons at 92 dB are probably my most efficient speakers.

Very happy with Legacy Audio Focus.

I have Zu Druid V’s with a prima luna HP.  Sooooo good!!!