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

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.

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. 

You can now find 2nd hand Avalon Eidolon Diamonds in good shape around $10k. Very coherent sounding and ticks all the boxes.