BEST SPEAKER THAT COMES TO LIFE AT LOW LISTENING LEVELS

You could try an equalizer or an integrated amplifier with a “loudness” button on it to amplify low frequencies at low listening levels. That’ll give you that rich sound at lower listening volumes. 

Since you have tried ML ESL hybrids, and liked them, why not audition a full range ESL, especially Quad 2912 and Sound Labs? I’m a Quad man, (ex ML - got tired of hybrids, YMMV) and I listen at such low volumes that my friends complain about it. So I guess that we have something in common there.

I can say from experience that Quad ESL’s work well with Bryston - I moved up to my own cost-no-object build, but Bryston still sounds good to my ears. Magnepan also fits with your 4B3, and might also fit well with your own requirements. I use a pair of 7B3’s for LF augmentation (Magnepan DWM).

And don't spend a dime on exotic cabling until everything else is exactly where you want it. Least bang for buck, most depreciation. IMO. YMMV.

Good luck!

Any Tocaro. Not just high sensitivity but they „excite aiir“  rather than pump air. Always wonder if you see membranes moving. Even at ear shattering loudness they hardly move. But just „explode“ at low levels. 
 

Tocaro from Germany. No marketing bs. Just decades old and highly refined and improved principles. 

Anything Tekton is great at low volume. Large and sensitive 

High efficiency horn loaded + EQ, either software based at streamer or analog piece between DAC or pre.amp and amp set to compensate for fletcher-Munson curve noted above. I often listen at low volume and have a Klark-Teknik DN-360 between DAC and integrated amp feeding Klipsch horns. Sounds very full and alive at low volumes. When I turn it up louder, simply disengage the EQ or turn boost way down. Works great.

With your budget look at Volti horn loaded speakers.