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

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.

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!

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.

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 (http://www.linkwitzlab.com/) 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.

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.