Used Speaker in the 15k range

I would recommend you audition a pair of Martin Logan’s.

They perform exceedingly well with classical, jazz, vocals, folk, etc. However they do require at least 3' of space around them.

Wouldn’t be my first choice for strictly rock though.

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!!