Best Late Night Low Volume Speaker

Bumping up the bass to compensate for lower hearing sensitivity to bass at low volume (i.e., compensation that mirrors the Fletcher Munson curve) would result in a more natural sound. But, bass energy is more easily passed through walls and other structure so it would also be more annoying to others in the house, and in that manner, defeat the goal of playing softly to avoid disturbing others.

If one must use speakers, instead of headphones, finding something that sounds decent for nearfield listening (a speaker that is not too bright) and sitting fairly close to the speaker would be another approach.

As you searching for loudness at low volumes, you must consider an analogy of : maximum clarity, transparency & above all, more than enough power to let the speakers effortlessly breath those qualities.
At this point of stethoscope analysis, the wiring loom is critical for the desired balance.
Now the story goes...
Back at 1998 I've trade my Soundlab Aura for the Proac Response 5 forced by a strictly personal & undue cause :
I could not get used to the way that the ESL speakers loading the room. It seems to me that the sound was stacked about a meter in front of them, while it was impossible for me to make them dissapear. It was like watching the music coming through two big open doors. The mind influense of those big panels playing music in front of me, was something that distract me as I could always hear everything coming from those "open doors". The listening pleasure then was replaced by amazement & awe about their unique & unsurpassed qualities : Extreme speed & transparency together with coherence, unbelievable inner detail together with body, weight & full amount of bass.
But a dynamic speaker, can energeticaly pressurize the listening space with a different kind of projection. A more visceral, more soulful vibe that engage my heart rather more than my mind. So, I've went for this trade-off, but never really could manage to surpass or even equal this speaker's qualities until today.
Yes, with the Proacs I was listening somewhat mudded, a bit uncoherent & about the half of what is recorded, but with more nerve & meaningful impact in my heart. I become aware of this preference as an unexpected paradox & consider it as a conflict between logic & emotion. (Brain damage ?) I cannot accept an acquired taste, I cannot learn how to like something, even if I appreciate & realise it's mastery of qualities, I cannot obligate my heart how to beat.
This Soundlab was my favorit ESL over Analysis, Martin Logan, Audio Static, Audio Exclusive & Acoustat.
I used to drive them with Counterpoint Natural progression & later with custom Futtermann 130watt monoblock. Then became apparent the true character of the pairing :
The speed & clarity, the vivid illumination upon the tiny shades of micro details, the enhanced sense of scale & especialy the unlimited HF air & the breadth of atmosphere on every recording were breathtaking. I know you cannot find a better OTL than Atmasphere out there, but I insist about the power reserve (over 100watt) at least with the 2 speakers that I mention here. I've tried accidentally my DIY monster Futtermann of 130watt with the Red Rose R3 (the predesessor of the Transmission Audio M1i) and I've found the missing link : a perfect synergy between them. A very special blending of qualities that many times makes me feel like the combination can defy the laws of physics. The holographic truthfulness was unbelievable to the point that after this session, everything else, was perceived by my brain as faulty, processed & adolescent empty, cold & sterile facsimile, without any credible illusion of the real feeling that can move the senses & satisfy the soul. In my trip to this hobby, this was the most memorable & delightful performance I've ever experienced in my home.
Now I 've moved on SET/Horns only for the adventure of the route & the haunting vision about a puristic direction where the brain swallows the music without guessing, or any kind of effort to synthesize & re-process the message.
But I surely regret.
The difficulties & spending of both money & courage to stand behind the project with integrity (full DHT valve of every rectification, regulation & every gain stage -even the active pre-pre and furthermore & especially the FR/BLH speakers) just don't compensate with the results as yet (after experimenting for 8 years).
So, I can't suggest you my custom OTL or the Red Rose R3 'cause they are not available any more, but I asure you both the Atmasphere (MA1 that I've auditioned in my system with the Proacs & was a heavenly touch of life) and the Transmission Audio M1i (that are available & as much as possible improved over the R3) are workhorses that can change the way of critical thinking & value judgement about audio components. This pair can open new paths of unexpected & new found pleasure as knows exactly where the heart of music is, and always exhibit precisely what performance the composer has in mind. Not a contest with usual hi-fi additives or hi-end perspectives. A treasure !
I wish someone could try the M1i with MA60. If this works, then by all means it could be the killer set-up of many mega $$$ pairs.

Best? No right answer, but my older Rega Juras powered by an Almarro 5 watt amp does the trick for me. I bought the Juras new with my Naim system years ago and repurposed them to bedroom duty when I upgraded speakers. Juras are great with tubes! My wife and I do lots of low level night listening before bed. Their dynamics at low levels never fails to delight me, especially with Jazz and acoustic folk or rock. They are supposed to be has-beens, but nobody told the Juras. They just keep happily singing - efficient rear ported floor standers with great base response, a smooth midrange, and wonderful tweeters that really let the Almarro shine. They are proof that good sound does not correlate to hi expense.

Another recommendation for late night, low volume: Air Tight MSM-1 Bonsai. And BTW, I would expect single-driver designs in general to do well at low volume, obviously with a good amp.

Yes, Tobias. My new Omega MaxHemp V2's are VERY revealing at low volumes, giving an easy, effortless sound. The amp is very quiet, almost zero noise from the drivers. IMHO the relationship between amp and speaker are key for retrieval of low level information and IN MY EXPERIENCE, high efficiency speakers help a lot with this.