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

For 70db & medium sized room
+ matching amp recommendation please
I've had great pleasure from my Monitor Audio PL100 II. Those are 6k new. If you go up the list to the slender floor standers you'll find speakers you could get well under 10k used.  I find the Monitor remarkable, and much better -FOR MY TASTE-- than Totem, Kef, Focal, etc.
You also might look for Aerial 7k. I listened to the smaller ones and found them quite good. 
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I didn't see in your posts what type of music you listen to.

I listen to mostly classical and jazz at relatively low levels, even at night while I'm sleeping.

Some time ago I discovered Meadowlark speakers which are sadly no longer being made, but for me provide exactly what you're looking for.

I bought (used) five different pairs: Swifts, Kestrels, Ospreys, Swallows, and Kestrel 2s.  I find them all to be wonderful, and you can usually find them pretty cheap.  I got the Ospreys shipped from Texas to New England for $1500.  They're gorgeous and I wouldn't trade them for many of the new speakers being made today.

 Good luck in your quest.
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.