Most coherent low volume speaker with very fast bass under 10k used?

For 70db & medium sized room
+ matching amp recommendation please
You can now find 2nd hand Avalon Eidolon Diamonds in good shape around $10k. Very coherent sounding and ticks all the boxes.
Some used or demo DeVores. A friend has the Orangutan 0/96 and they are not screaming speakers, they seem better suited for low-level listening. 
If you can find a Sanders ESL used, it will likely be under $10,000.  This electrostatic speaker is absolutely stunning in its sound at all frequencies and the bass will be right up your alley.  If you see one used,  you better move fast on it though.   VERY FEW owners are ever selling a pair.  That should tell you something.  These speakers are absolutely non-fatiguing.
I didn't see in your posts what type of music you listen to.

I listen to mostly classical and jazz at relatively low levels, even at night while I'm sleeping.

Some time ago I discovered Meadowlark speakers which are sadly no longer being made, but for me provide exactly what you're looking for.

I bought (used) five different pairs: Swifts, Kestrels, Ospreys, Swallows, and Kestrel 2s.  I find them all to be wonderful, and you can usually find them pretty cheap.  I got the Ospreys shipped from Texas to New England for $1500.  They're gorgeous and I wouldn't trade them for many of the new speakers being made today.

 Good luck in your quest.
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I've had great pleasure from my Monitor Audio PL100 II. Those are 6k new. If you go up the list to the slender floor standers you'll find speakers you could get well under 10k used.  I find the Monitor remarkable, and much better -FOR MY TASTE-- than Totem, Kef, Focal, etc.
You also might look for Aerial 7k. I listened to the smaller ones and found them quite good. 
I have the Rockport Aviors and I find them very coherent. They play particularly well at low SPLs. However, I my view, they need good quality, high current amplification to bring out the best in them, especially at low volumes. As such, I second the view that you should focus as much on amplification proposed to be used as much as the speakers - the two are not mutually my view
"The amp used to drive the speakers is more important to how fast/well controlled the woofers etc.. "

Couldn’t agree more amplification and cables determined the quality of the drivers control ,it’s better to invest your fonds on good amplification and cables with decent speakers than buying suppirier speakers that you can’t afford the proper amplification to drive and control them.
Haven't had my db meter out lately however I have found that my Sonus Faber Olympica II's ($10k new, much less used) are remarkable at most moderate levels along with a small M&K powered sub crossed in at 50hz... blends seamlessly for my ears after some careful adjustment and placement (highly subjective) and tuning of all the speakers in my listening room which is my family room off the kitchen.... Since I do have a wife who goes to bed early, I tend to play that system at lower levels except while cooking, cleaning up or entertaining.  I also love the Olympics III which are 1db more efficient and go a few hz lower that the Oly II's... all with stunning imaging and clarity... I must emphasize that for critical listening placement for all 3 speakers is critical relative to the room, your seating and its furnishings etc.  Of course they will also reveal the quality of your system components cables etc.   The Sonus Olympica group I,II, or III's need to have air around them while my old sub feels/sounds best corner loaded in my particular  application.  Bass is deep, quick, defined and effortless sounding...

There is no substitute for listening in your personal application.
When one of my audio pro buddies was over listening he kept asking if the subwoofer was turned on because if you didn't see it sitting there you could not easily be aware of what it was doing versus the Oly II's ...  No I am not a rep for Sonus Faber, I just happen to love them!  Enjoy the music as much as the quest.

I second the suggestions of Vandersteen speakers (I own 2 sets of Vandersteen 5's myself), for low volume performance I was even more impressed by Linkwitz speakers ( and he's got even more selection now. I met with Siegfried Linkwitz 10 years ago and even rented a vacation property from him for a weekend, which gave me the chance to listen to one of his personal Linkwitz systems for the weekend.  I was very impressed with how well they operated especially when I wasn't in the sweet spot and focused on critical listening.
Raidhos!! Very fast bass and perform excellently at low volumes. Don't let the small drivers decieve you, these speakers play low.

Dealer disclaimer: I carry raidhos and would be happy to discuss further. PM with any questions. They are a remarkable speaker.
The amp used to drive the speakers is more important to how fast/well controlled the woofers etc.. will respond.  Hard to find gutsy amps these days that can grip a speaker and control it!

The "fast bass" thing may exist in certain theories or somewhat useless measurements (is metal "faster" than paper?…uh…no…) but is not relevant to the ACTUAL sound of good hifi…assuming "all speakers" have this "time smear" issue is silly, as is the claim that 85 DB is "far too loud." For what? Sleeping? Level is relative to the listener's comfort as few listen with their speakers stuck to their heads (except headphones, another topic), and luckily few believe the "fast woofer" theory as it applies to actual real time listening and can enjoy great sound without worrying about their "slow" woofers in either their main speakers or in subs…or swarms of subs found by many to be both musical and coherent (including in some more sophisticated live concert environments). Balderdash I say! Luckily nobody listens to theories (except the ones adopted by the people who designed the gear), but they may actually listen to music.
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Totems create dynamic tension and contrast at low and loud volumes, although instinctively you don't feel compelled to blast the music when realistic dynamics are created at less than ear bleed levels!

Well, i mention very fast Bass because this is the weak spot of all speakers. High inertia of the large bass driver or time smear with multiple small ones destroy coherence and introduce timing problems. 85db - where all frequencies start to appear even - is far too loud. I think there is also a market for excellent low level speakers that are optimized for 70-75db. If you play that speaker louder and bass and highs get too intense so you turn the level down again - well that can only be a good thing for our ears.
Low volume listening isn't necessarily compatible with higher level listening as a design conundrum exists…Fletcher Munson et al...powered subs ("fast" bass is a term I find meaningless) can help if low level listening is what you want…you can turn the relative level up…who knew? Horns at low level better be pointed at your head, but I still think any speaker that somehow exceeds at boosting the frequencies that otherwise drop off at low levels can over boost those frequencies when you turn stuff up. Get an equalizer maybe as at low levels you don't get proper musicality anyway, so who cares if it adds crap? 
I would like too add that I like wide dispersion speakers. Think ATC SCM50, with the coherence and speed of a quad 57, and dynamics of horns.
((Most coherent low volume speaker with very fast bass under 10k used?))

Vandersteen Quatros
Adjustable bass = clear in room bass response.High Pass allows the amp from heavy lifting = dramatically lowers distortion further improving transparency and clarity at all volumes.

Vandersteen 5a
and you can EQ for the room
amp optimized to drive the built in push pull subs free you to choose many amps for top end
nice pair of 5a for sale in agon now

There’s nothing like a full range electrostatic loudspeaker for coherency imo. As you say "low volume", a pair of QUAD’s will do quite nicely. Restored 57’s are a perennial classic and favorite, but also the model 63 and it’s descendants, the 998/9 and 2805/2905. All are well below $10k, and you’ll even have enough for a pair of great subs to augment them. Few are good enough to keep up with the very "fast" QUAD bass, the best being the GR Research OB/Dipole Sub, which is a perfect match.
It's right there in your name: Zu. Get the best Zu speaker you can afford and a DECware amp to drive it. Very good at low volumes and fast.