Used Speaker in the 15k range

I would consider 
Vienna Acoustics 'The Musik'
Meridian dsp 8000
paradigm persona 7f
Kef Blade
Yamaha ns-5000
4 speakers that can be found used for around $15k that I have already or will in the near future own as I figure out what my idea of the perfect speaker is. 

You can get new Quattro CT for your target $$$

putting together system right now for buddy: PS Audio DAC , Aesthetix Signature Preamp, Vandersteen 5 high pass amps, Quattro CT in zebrawood....

fun

enjoy the music and your journey 

If you're going to upgrade everything then take a look at Kii 3, Dutch and Dutch 8c,  Genelec 8361A. 

MAGICO,Vimberg or Rockport jump on a pair ASAP!!

If you poke around a bit you can get a pair of JBL M2s for right around there.  I would have a hard time believing you could get a more accurate, better sounding pair of full range speakers for the same price.  I looked at a lot of different speakers thinking the M2s were out of my price range.  Then I did some digging.  It's worth asking.