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

The character of the B&W speakers is pretty unique and colorful.

The amps less so. Those are more neutral.

If you like the sound as is, keep what you have. Remember that good room acoustics can make your speakers sound louder and taught.

If they won’t play loudly enough, change your amp, and consider class D solutions.

Best,

E

OK, who am I to give advice when I picked out my preamp and amp before choosing my loudspeakers?
Here is the conundrum. Many of us (myself included) believe that source and electronics and cabling are more important than choice of loudspeakers. This drives the other camp bat-shite crazy. That's fine, we are all entitled to our own opinions. 
My experience from 45 years in the hobby is that with everything else carefully selected, a very humble set of loudspeakers can sound amazing. On the other hand, the finest set of loudspeakers imaginable will always sound like crap fed a crappy signal. 
Sounds like I am arguing for your course of action and not against, doesn't it. Yep. I am. But now you have to choose your loudspeakers carefully. Anyone and their brother on this crazy Board can throw out suggestions for relatively efficient loudspeakers but they are all going to sound different in your room and may or may not suit your preference. Asking here for suggestions is worth maybe 5 cents and a used piece of gum. 
I happen to have spent ten great years loving a pair of B&W Matrix 805's I bought new in 1998. Your 705 S2's are basically the same loudspeaker with technological advancements. That means they image like champs and have a very pure sweet midrange but don't give you the immense wall of sound that only larger speakers can provide. For example, I remember taking the Mapleshade recording of The ARC Choir to my retailer and with the larger B&W floorstander's I heard the complete choir spookily in front of me while I could not coax the same with the 805's. 
But....you have not given us a budget and you have not told us what you like in the way of speaker attributes. Every loudspeaker on Earth has strengths and weakness and character traits. No exceptions. 
My first piece of constructive advice is decide your budget and which speaker characteristics you like. Then call Galen Carol and ask him. 

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.