Used Speaker in the 15k range

If you like lifelike timbre and musicality and dynamics with bass extension to 22K and a beautiful cabinet (made of 21 layers of wood) check out the Gamut RS7i (Ruby) up for sale right now. Extremely musical and possibly $15K would buy them. I know the seller and the care. Speaker retailed for $42K when new.  
gwalt

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.