What makes a speaker sound great at low volume?

Most of the time I hear music at a low volume (wifey, apartment, ....). 

I am looking to upgrade my current speakers, but in my market scanning I would like to understand, if there are certain “metrics” to look for, before I start going to stores for listening. 

Any advice? 
Yes - look for a speaker that sounds good at low volumes. I’m not being sarcastic. One of the many mistakes I found that people make is doing demos at loud volumes and then deadening their ears. I think ears react to sound like pupils to light. If you go into a dark room - the eyes open up and then you can see. If you start listening at a volume that’s a bit too soft, your ears open up and the volume gets more normal. Then if you kick up the volume just a smidge, it sounds dynamic and full. If you listen at low volumes you also might consider an amplifier with tone controls to allow you to add a bit of bottom and top to the mix when you’re listening softly. I’ve worked pretty hard at putting together a low-volume system and there’s no real secret, IMO. Demo the speaker in your home and see if it sounds good at low volumes. That’s pretty much it.
In fact - try the low-volume experiment with your current system.  Start a little too soft, listen for a few minutes until it feels comfortable and then turn it up just 1 notch.  Maybe you're ok as is and it's just your listening technique.  That saves a few bucks.  Good luck. 
One thing I'd say you likely Don't need (necessarily) are expensive, overdesigned drivers. Even ordinary paper cones are quite competent and capable of very nice combinations of musicality and detail/resolution. An exception in your case might be planar/electrostatic/ribbon designs. These, if they fit your budget, can offer even more separation/clarity/transparency and so forth.

Where I think the most design money must be spent in your case might be on the crossovers and it will be a challenge for you perhaps to find an example with nice, ordinary drivers combined with excellent (expensive) crossovers at an affordable, or entry-level price. Of all the design components in a speaker, I'd say generally that the crossover is the single biggest obstacle to good sound. But this is only because most manufacturers end up giving it short shrift - they likely could do better, but they simply don't. Most people don't realize just how important crossovers are to the kind of sound they are looking for and most makers seem content to let that sleeping dog lie, if you will, rather than try to educate the buyer. But for you, crossoverless designs are at least something to consider and to go and hear, if you have the opportunity. Single-driver designs stand to save you some big bucks and a long time looking by doing away with the crossovers altogether. The only real catch to look for might be the relatively rolled-off highs since no single driver is truly considered full-range.Crossovers may take the single biggest hit to sound quality in the overall resolution/clarity/transparency/coherency package - perhaps especially the coherency - whether it be dynamic, tonal, vocal intelligibility or spatial coherency.

All that I'd add is that while you'd want speaker sensitivity to be modestly high, you certainly don't want it too, too high, or you might be dealing with a system hum that is most annoying at low volume levels and that you then can't get rid of without swapping out gear. In the low 90's might be ok, but 100+, almost certainly not.
In terms of what metrics to look for, in my experience the speakers with very high sensitivity, >96dB, combined with high load impedance, >10ohms, tend to sound more detailed and full at lower listening volumes. I have a pair of single-driver full range horn loaded speakers in my upstairs converted listening room paired with a 10wpc integrated amplifier and its perfect for (almost) near field low volume late night listening.
Crossovers are the most ignored, but most important consideration, IMHO. I recently went through the same process you’re going through now. I ended up with the 2 way Joseph Audio Pulsars, which provide everything I need ... great crossover (a Modafferi based and JA modified infinite slope), fabulous sound at low and high volumes, low distortion, and great looks. Couple them with a good sub and you’re good to go. BTW, I went from full range B&Ws to the 2 way Pulsars, and can say that I much prefer the more coherent sound of the 2 way with a sub than the full range speakers. 2 way speakers can "disappear" in a way that full range can’t. Of course, your room size will have a major impact on which way to go as far as speaker size is concerned.
kalali is correct, omega speakers have 98db sensitivity, like my rs8 supercone, high output alnico 96db ? Then get a nice tube  power amp.
Buy a pair of Quad 57's! Either in good working condition or refurbished. They will provide all the air and detail at low volume that you could desire! You will be very happy! I own two pairs! And a good amp match for them would be a Quad 303 (I have one, along with a 405 and a rare pair of the 50 Mono's).
I love the sound of my Ref 3A de Capos at low volume detailed

using with a 4wpc decware amp
You should go listen to any LS3/5A type of speaker. Spendor, Harbeth, Stirling , etc., etc. make this design!
My former Krell 600 - Dynaudio Consequence combo was hopeless on low volume. It would have been better to listen to a small bookshelf system. It was only on higher volume that the wolf came out of the lair, so to speak. Music started sounding great, and even glorious, but only at very high volume. My present Atmasphere MA-1 - Audiokinesis Dream Maker combination offers better sound at lower volume. Even though I could have asked for even more, in this respect. Here also - even if these amps run fairly "flat" in class A - I find that to get the sound right, I need to turn up the volume. I think that getting more sensitive speakers is the way to go, but it is not the only answer.
Properly designed loudness controls, Schitt Loki, or another eq ( in the tape monitor loop ), would answer your concern. Headphones are another route to take. I am not a scientist, but been in this hobby for so long, these are my experiences. Speakers load a room differently at various volume levels, no matter what equipment you have, and our ears are quite unique instruments. Happy Fathers Day to all the pops out there ! Enjoy ! MrD.
@mrdecibel I agree . Hi end vintage gears was equipped tone control
loudness control etc, so this is problem with all hi end which you
see on the market now. Is very rare to see all new amp, with any
tone adjustment,  so this issue is depend from speakers . Sensitivity is
absolutely no important. take a look on the freq . responce---
if it is go  slightly up in midbass or bass   it will be play good on the
low volume,
"What makes a speaker sound great at low volume?"

The "Loudness" switch on my McIntosh C22 preamp...
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I use the tone controls on my McIntosh C2500 preamp at low volumes.  Does wonders.
I am thinking of getting the Luxman 509x for my office and one of the big reasons for it is the Loudness switch. It would help big time when I am working in my office at 3AM.
While some if not most of the above comments certainly have merit, I think one over looked crucial factor is the speaker placement itself.
Many audiophiles change things before really getting the most out of what they have in the first place and speakers are certainly one of if not the most important piece of equipment.

My speaker Dynaudio Special 25,s certainly aren't the most sensitive' I found that once I learned to REALLY set up speakers the low level listening details became more aparent with low level listening much more rewarding. Low level listening for me is early mornings when I'm preparing for class, but when I get home my listening volume goes up to high 70db,s to mid 80,s and the louder you play the more crucial speaker placement becomes. 
Another thing that really helps is an amplifier with a properly designed, "stiff" power supply".
There are many variables, starting with cables .
since I upgraded to the Verastarr cable line I feel that using far less dielectric material ,as well as metal purity and Cryo treatment 
all add up to a cleaner presentation .
that is just one example , the amplifier, preamplifier quality 
are seperate factors  ,that being said yourcable s carry your signal.
your sourse record, or digital is where the Whole ball game starts 
once that signal leaves it cannot bemadeupdown stream .
sice by far your Loudspeaker gives the most distortions in a 
factor in % , 1-2-3 or more vs.001 for a amplifier on average a good one from 20hzto 20,000khz. Quality counts even more so 
And complexity of the Xover , speakers sensitivity , and the ability 
ofthe Amplifierto drive properly and have dynamic head room leftover for when you hit dynamic peaks they are not compressed 
on peaks demands can call on 10x thst then on a soft paggage 
allways buy bigger Quality power then you need  to ensure 
ultimate fidelity. There are many other factors I didnot even get to 
such as keeping all connections ,including power cordsclean.
the recordings the self will dictate what you hear.Room sound
absorption,isolation underequipment ,including a qualitystand. The room can have a lot to do with what you hear.
proper speaker setup. On the Cardas cable website they have a good setup guide line. Myself having owned a Audio store in Europe learned a lot about how to get many systems to sound far better then systems 3x the cost just by following all these guide lines.as thesaying goes ,yoursystem isonly as strong as its weakest link  !!  Enjoy Your music !!
I’d combine chayro’s (first reply of the thread) and dweller’s posts.

If you are really stuck with very low, middle of the night kind, volumes, "loudness" switch may surprise you. dweller says McIntosh and there are a few more, depending on your budget. Luxman, Accuphase (I am almost certain, but check it). Yamaha also has one although that one, in my experience, is not as good. It works in a different way. If you get intrigued about the "loudness" switch, there is a thread on Audiogon that has a few descriptions of it. https://forum.audiogon.com/discussions/how-loud-is-too-loud

Now, you asked about speakers and a few of us are throwing amplifiers at you. Sorry.

Good luck, listen low.
Lots of things. My speakers aren’t the most effecient and require a big amp. My system sounds very good at low volumes which is how I do most most of my listening. I’ve done some tweaking to lower the noise floor which is what I think has worked for me. Start with clean power, dedicated circuits, quality receptacles, quality cables, and cable placement. Nice rigid equipment stand, isolation devices between your components and shelves. I think it’s getting rid of the noise to start with. Speaker placement, your listening position and room treatment also play a big part. Guess I’m saying it’s not just looking at a pair of ultra efficient speakers because it can be done with not so effecient speakers as well. It takes time, experimenting and not being afraid of tweaks, such as fuses and etc.
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My rec room speakers are a pair of vintage DCM TimeWindow 3's.  I set the volume on my McIntosh 31V A/V control center to an acceptable level and then use the 31V's EQ and McIntosh MC2600 power amp gain controls to tweak the sound.  This system is all analog.

In my music room, I have a NAD M12 and NAD M22 v2 pushing a pair of Tannoy dc8ti speakers.  This system is all digital, except for my Techniques SL 1600 MK II turntable.  I use the M12's volume control, which works OK - but , the treble and bass controls are no match for a good EQ like the 31V
I don't know if particular design attributes contribute to, or detract from, low level listening.  But, as a general observation, the best sounding systems at low volume tend to be high efficiency designs, particularly horn-based systems.  It may seem odd, but, those massive horn systems play better at low volume than at high volume (this might explain why so many Japanese audiophiles are attracted to horns even if they have neighbors that are very close by).

In part, high efficiency allows for the use of low-powered amps.  Low-powered amps, particularly tube amps, sound very lively when played at modest levels, while high-powered stuff tends to sound dead at low volume.
chayro: Love your first sentence ...

OP: I don't know if you're considering vintage speakers as well, to solve your low-level, listening issue, but I have a pair of Kef R107's, and they produce a wonderful sound-stage no matter where you are in the room. They are supplied with a small EQ adjuster that is electronically, part of the crossover. At low volumes I just boost the Contour adjustment dial. (This is the only time I touch the contour dial). This seems to add what's required to cover all the necessary frequencies, and the music sounds great.
Most people seem to like the 107s, but some just don't. They're a little different.
Anyway, just my two cents ...
IME, active, near field, studio monitors are probably the best bet.  Get the x-over ahead of the power amps and you’re most of the way home.  The best examples are quite expensive, but even many of the modestly priced units are very good at lower SPLs
     Interestingly, I noticed an unexpected improved low volume listening performance when I switched amp type from an Aragon class AB to a class D amp of about the same wattage.  The improvement was significant even though I was using an older pair of Magnepan 2.7QR large panel spkrs that are fairly inefficient (87db @ 1 W).  
     I'm familiar with 'loudness' controls and would equate the improvement to their affect; a perceived increase in the bass and treble to compensate for the proven reduction in perceived bass and treble when listening at low volumes.  
     What I found especially noticeable and beneficial with the class D amp performance was that the proper bass and treble presence maintained its proportional accuracy at all volume levels, from soft to very loud as if there was a 'loudness control' continuously engaged. 
      I cannot verify that all class D amps share this ability, however, I currently own 3 different brands of class D amps (ClassD Audio, Emerald Physics and D-Sonic) and they all share this ability or characteristic. 
     I would also agree with the several other responses on this thread that  stated moderately high efficiency speakers would be beneficial for improved low volume listening performance.

No crossover. Tube amp directly driving the woofer...

+1 .. my Decware/Omega pairing continually surprises me with the level of detail and dynamics when listening at low volumes levels. 
A few people have suggested ESLs, and I'm pretty sure they'd fit the bill. I had ESL's with a McIntosh set-up, about 40 years ago, and they were very good at low volumes. Mind you, the MX-113 tube pre had bass and treble controls, so that helped. They also had a dynamic low-frequency driver ...
Many thanks for a lot of inspiring input. I have enjoyed reading the responses including the recommendations for other ways to improve SQ than replacing speakers.

My selected take-away for now is
- go for high sensitive speaker (above 92 DB)
            - eventually go for Horns
            - eventually go for studio monitors (or mid size speakers) 
- add EQ in some way, either via DAC, Pre-AMP or room correction
- replace current AMP with a Tube-based AMP

I will start with the speakers and move down the list - and replace my DAC at some point. 

Many thanks. Michael
My experience shows me that good interconnects and especially an upgraded power cord (nothing crazy here. Say $50 for each set of phone plugs to get decent quality plugs and say $75 for the power cord. The Pangea cord from Audio Advisor is very good). People tend to go crazy here but there is a point of diminishing returns of dollar to sound quality. My point is being-to recover as much detail as possible which fleshes out the music at lower levels and does improve efficiency as well. Speaker placement through trial and error, assists greatly to produce a solid sound stage where getting the tweeters at ear level is important.  Smaller speakers, say between Dynaudio to Def Tech 800s are great with a quality subwoofer.  
Chayro gave you very good advice. I listen at low volumes because my ears are sensitive. It's been Magnepan or Quad for nearly 50 years (2905's for a dozen years now, and no thoughts of, or lusts for, change).

Magnepan or Quad. Nothing else comes close.
High efficiency (high nominal impedance and sensitivity). Other parts of the chain can have an influence but efficiency is the primary factor.

What makes a good speaker sound good at low volumes ?
 "" A good Speaker"" and listen to it !
From my understanding Frequency Response changes with Volume.
This is  why you see Loudness contour Switches.
i remember Yamaha amps had adjustable Contour Control.
I remember reading The Fletcher-Munson equal Loudness curves.

I auditioned speakers on modest dollar for just this issue, with my wife who has great ears but hates higher volumes. Mostly classical jazz and acoustic. Walked into shop expecting to buy the KEF's, came home with Rega RS3. Elegant sound.  Adding an SVS sub eventually added the bass fine tuning at lower levels. And my modded Denon pre has great loudness control if needed.
A friend's Vandersteen II's with great electronics sounds fine on lower volume acoustic stuff also. But as others said if I had money and space for them I would have Quads or Maggies.  Maybe later.  Agree that placement and listener position huge. Near field likely the way to go.
omega speakers! they are simple fantastic for low level listening. and no slouch @ 11 either (:
pair them with decware or linear tube audio and you'll need spend 10K to do better, just sayin

Its all about timing. At low sound levels the ambient room noise dominates and the electronics and speakers move off their ’sweet’ spot. But you can still sense the ’grip’ of the music and vocals if the transient timing is preserved. The three Casablanca audio clips in this paper are particularly enlightening:
So, i was able to supplement my system with a TakeT BatPro2 super tweeter (which is a just a phenomenally fast tweeter) which maintains the energy of the transients to make everything audible and makes low level listening very enjoyable.

Two suggestions that I have owned, and sound really good at low volumes: Monitor Audio GR10's, matched with a NAD 326BEE - modestly priced combo that sounded fantastic (still regret selling) - and Ascend Acoustics Sierra 2's (which I still own and love), paired with a Rga Elex-R.
That is one of the tests I do on a resolving system is test for low volume.  I find very efficient speakers with class wattage (zero feedback) tend to do the best.
The Fletcher Munson curve is a reality that cannot be avoided, and although efficient speakers (especially horns) do "seem" to sound better at low volume (I use Klipsch Heresy IIIs at 99db efficiency with a 12 watt per side tube amp) they can’t run from old Fletchmun...if you can live with single ended ins and outs from your preamp to amp, get a Schiit Loki...an exceptionally low noise inexpensive 4 band EQ that seems made for this sort of thing...I don’t use mine that often, but since it doesn’t drag on the signal (utterly transparent when "out of the loop") it’s always there when needed, generally to add sparkle to dull sounding recordings or kick Fletcher Munson in the gonzagas.
I find Fletcher Munson is too base heavy for my ears.  Perhaps the equal loudness contours solve that problem.   Perhaps the 12 band jriver media center eq would be useful to tweek the contour settings for my specific sensitivities. Haven’t tried it yet.  Not certain how well one would overlay the other. 
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I agree with what some others have said regarding the Quads and Maggies. My first love of speakers was my Quad ESL63s. Even at low volumes, they captured the width, depth and detail of a performance, without leaving your head numb. After many years of happy listening, I had to give them up when moving into a home with a somewhat smaller listening room.
After moving back to a larger space, I considered getting another pair of Quads, but decided to give the Maggies a try. I’m now quite enjoying the little Maggie 1.7s. paired with my ADS sub. I listen mostly early mornings and late evenings at very moderate to low volumes and (like the Quads) the stage remains large and detailed, even at low volumes. Even though they’re not what you would call real efficient, they do very well, driven by my Rogue, Cronus II Mag - 100w per side, int. tube amp.
If you have the space and the budget - definitely would fit the bill, especially if you listen to jazz, light pop, bluegrass or anything with real instruments and good vocals....Jim

Mid base is great with my 1.7s, even at low volume, but do need to run a sub to get any impact below 50 Hz. Setting the filter on the sub down to 50 or 60 Hz and getting the volume and polarity right, makes a good and seamless interface with the Maggies - at least in my room.....Jim