Recommendations for speakers that sound great at lower volume levels.

Every single loudspeaker you will ever own will do the same thing. Actually, it is not the loudspeaker. It is your ears. Better yet , it is everyone's ears. Google Fletchur-Munson curves.
What you need is loudness compensation. Older preamps usually offered a single level loudness filter but in the "No Tone Controls" era the filter was dropped. Consequently, if you want a recording to sound right you have to play it back at the volume it was mixed at which depends entirely on the mastering engineer. Loudness compensation will give you a lower volume that the recording will sound "right" at. In the digital era this is easy to do without adding distortion and ruining the image. Those with room control can program their own loudness curve into one of their presets for times when they do not feel like shaking the shingles.

A great low volume speaker is a pair of stand mount Fritz Carerra BE Speakers.  I recently picked up a pair and they are fantastic.

I listen to some Altec Lansing A-7 500's that are 16 ohm, driven by Decware, 6 watts/channel.  Very sensitive speakers that sound unbelievably great at all volume levels.  I have found some things that have the sensitivity is 95 and as high as 115 db.  Whatever it is, wow, they sound great.  I have tried Caintuck's open baffle and found them very pleasing at low volumes and Blumenstein's.   That said, I had some Maggie's in 3.5R's that were similar to your Harbeth's, it was like they were asleep and not nearly as dynamic until you cranked some heat to them.  Then everything woke up and it was incredible.  I believe that both Nelson Pass and Steve Deckert would say about their products, "good from the first watt".  Might try to find something more sensitive for speakers.  I had a long discussion with Steve Deckert when buying the amp and as he said, "nothing beats a single ended triode for sound, unless you are drinking some beers and want to really crank it".  Another option, as those are very nice speakers, I have had some good luck with a high current amp (not watts), driving some Linn LS35A's.  Another option is to reach out to Harbeth and explain your issue, see what they recommend.  I am a collector and not a seller.  If you do something, I would keep what you have and add to it.  It is so much fun to play new combinations of speakers with different power and input.  Good luck. 

I have also noticed great low level, balanced sound with my Salk Songtowers. It's as if I have a subtle loudness switch on all the time. This is regardless of the room they're in and the electronics.

My system has outgrown the Songtowers so I'm looking to upgrade along the Salk line for that reason.  

I would think the more efficient the better.

I wonder why no one mentions Focal.  I tried to like the Aria 948’s and even those were weak on bass.  I did think the tweeters sounded good.