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

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.

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.

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