Best small used monitors under $1000

Silverline  Minuet, Elac UB5,  and if you are willing to go just a bit over budget, the KEF LS-5o is among the best around at any price in a small room.(That is what I use in my 9x11 room with great results) Any of these could be driven to good effect with modest, but carefully picked out electronics. As you know, when you are assembling a world class playback system at any budget, make sure to select the best source components you can afford. A lousy source will make even megabuck efforts sound not nearly as good as they are capable of..

Let's talk about all the components in the system not just speakers.
brian

If you go used

Proac tablets

Proac response 1sc great if you can find a set most people keep them

sonus Faber has a bunch of nice small speakers that can be had under a grand now consertino domus comes to mind

ls 3/5 as people mentions but they need more power then most small speakers

Monitor audio and Dynaudio have some wonderful speakers under a grand too

That's a start so many more 

Some great suggestions here. Another is the Eminent Technology LFT-16a, a magnetic-planar/dynamic woofer hybrid that retails for $1250.