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

+1 for ATC 

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.