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

For 70db & medium sized room
+ matching amp recommendation please
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

((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.

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.