Best small used monitors under $1000

In that Price Range there is nothing better than active studio monitors.

JBL LSR 305 + chord mojo + tablet/ smartphone or chord poly

option 2

Neumann KH 120 AW (white version) + Ayre codex + sotm sms100

use single solid core cables only like audioquest, dnm, vovox

get good Stands 

The Wavetouch Grand Tetons are superb and compete with virtually any speaker made - can be found used for <$1000. True "giant killers!"

Second choice would be the KEF LS50 - without question.

Kef ls50 lack dynamics and need expensive amplification.

Trust me - Jbl lsr 305 - very good dynamics that some 10k speakers are unable to deliver. The Neumann kh120 is 10% better in sound quality, better build quality (cast aluminum), metal grilles, made to last forever!
chord mojo DAC - the very best if you listen to low Bitrate stuff like webradio, Spotify or MP3 

Dynaudio DM 2/6. The DM 2/8 is what I use really smooth in my opinion or the ZU Audio ZU cube are great.

KEF R101.  I have two sets in two different rooms.  I bought them new in the early 1980's.  They sound wonderful, and get better each year.  I see them from time to time on ebay and other used resources.