Can you tell me why I didn't like the Reference 3a MM's?

I have looked at the Zu’s as possibility. What about the 12 ohms impedance? Is that an impedance to good sound with my amp?

The smaller Zu models which are 12 ohms (Omen MkII, Omen Bookshelf) are rated at 97 db/1 watt/1 meter. While the XA25’s power capability into 12 ohms will presumably be about 1/3 less than its capability into 8 ohms, that reduction should be more than compensated for by the relatively high efficiency of those speakers. But having no experience with Zu speakers I can’t comment on their sonics.

Also, as Mapman indicated small size, efficiency, and deep bass extension tend to trade off against one another. So a concern with those Zu models may be the adequacy (for your purposes) of their deep bass extension. And I would not judge that by the published bandwidth specs, in part because the bandwidth specs for those models don’t have any +/- tolerance indicated, and in part because we don’t know how much placement-related "room gain" such specs are based on.

Good luck. Regards,
-- Al

Here is the conundrum. Many of us (myself included) believe that source and electronics and cabling are more important than choice of loudspeakers.

Well this perspective is the hardest to correlate with any measurements anywhere. As some one who builds speakers and has recently gone through a few rounds of amp changes this does not align with my experience at all, but it does again show that you can train your ear/brain mechanism to ignore some signals and be sensitive to others. I see it all the time at shows.

I have good source.
Metrum Onyx with a Kitsune tuned Singxer DDC
Audioquest speaker cables - the $18ft ones
Cardas and Audioquest interconnects.
MPD on a linux Raspberry Pi
E.A.T C-Major turntable with a Black Cube phono amp.
Aric Audio Unlimited preamp
Pass XA25 amp

I had a Odyssey Khartoga. Good amp, but I felt it was my weak spot. So I got the Pass.

I really need to stick with Bookshelf, as my "listening room" is my garage and the speakers are about a foot off the ceiling and wall for protection.

What do I want? Magic! I want it all! I do have a subwoofer, B&W to compensate for low end. I want good midrange, imaging, tonal and clarity. I want to go "Wow, that sounds really good." at times. Which I have now, until I turn the volume down. But as I get older, my wife and I both tend to listen with the volume lower. Sure, I still crank it up every now and then, but that is getting further and further apart. So I am hoping/thinking that a more efficient speaker would just plain sound better at a lower volume.

Most reviews of the B&W agree, they are really good with the volume up. Maybe less so at quieter levels.

Budget - up to $4k maybe, if everyone is raving about the speakers.

I am a bit surprised at your reaction to the Ref 3 speakers, as my experience with them is quite different than yours.  I ran a pair of de Capo i's for several years with a 35 W tube integrated (Primaluna Dialogue One), and I loved the sound.  They were very articulate IME, somewhat reminding me of electrostatics with their jump factor and delineation of musical lines (attributed to there being no crossover element on the woofer/midrange driver).  Regarding volume, one downside of the amp/speaker pairing in my system and room was that I couldn't get the volume knob up past ~10 o'clock, because things just got too loud.  This made it a challenge to get volume level just right, since the pot wasn't as finely sensitive at its lower end.

I actually tried a pair of B&W 805's in the same system/room and decideldly did not like them.  They just didn't have the life that the Ref 3's brought.  Maybe this is all about amp/speaker interaction?  Or room acoustics and positioning?