Speaker upgrade for classical music

If you are content to sit in a rather narrow sweet spot, then consider highly directional (focused) speakers combined with phase management software to widen the soundstage and separate the instruments. The goal is to minimize room effects and maximize fidelity to the original recording, while bringing out details and also creating the illusion of a wide, holographic soundstage.

Speakers (more to less expensive, all these are hybrid electrostatic except the KEFs):
Sanders 10e
JansZen Valentina (floorstanding) A8
JansZen Valentina (floorstanding)  P8
JansZen Carmelita (stand mounted), active or passive
Martin Logan (various models)
KEF LS50

Software (more to less expensive):
BAACH4Mac audiophile version
BACCH4Mac intro edition
HAF

The JansZen speakers and the Sanders are ordered direct from the companies. JansZen offers a 30 day in-home trial (I think Sanders does too). But don't do this unless you're pretty sure they seem right for you (since re-boxing and returning speakers would be rather inconvenient).

For your budget, I'd suggest the JansZen Carmelita with the BACCH4Mac intro edition (then later upgrade to the audiophile edition).  The Carmelitas are not only less expensive than the Valentinas, but also would be easier to box up and return than the larger floor-standers if you are not satisfied. If these interest you, then maybe talk to David Janszen about the Carmelitas + subwoofers versus the Valentinas without.  My thinking is that the main advantage of the Valentinas is deeper bass extension, but you can get that from the subs (which also may make main speaker placement a little more flexible). I'm not sure, though, if JansZen active models are designed to integrate with subwoofers (or software like BACCH). If not, then go for a passive version with your existing gear. JansZen is the Ohio distributor for BACCH software, so they should be very familiar with how it interoperates with their loudspeakers.

Sonus Faber

@audiokinesis makes a good case for forward and backward firing speakers. I note that panel speakers, like Magnepans and ESL's, do that naturally. Without phase control of course.

I like that sound very much. A good complement is record or book shelving behind the speakers to diffuse the sound, creating a wonderful ambience. 

Having read the entire thread, I will comment only on speakers I’ve heard. Agree on Harbeth, but also agree with comment that they are a bit too smooth on the highs, lacking some detail. Own Totem and they are more like the instrument is in the room - like a live performance. I find upper tier B&W speakers harsh and tiring in the treble, but some see it as precision. I prefer the KEF products at comparable price points. To me they sound more natural. A brand no one has mentioned is Aerial Acoustics, based outside Boston. I’ve owned their Model 8B, no longer made, for a couple of decades. The newer Model 7B is also excellent and full range, but Aerial speakers need power. 
I concur with the idea of an amp upgrade. And would encourage you to audition before buying, especially with an 8k budget. But you could spend a good deal less and do fine with KEF or Totem. Also, I agree with view that tone and voicing of instruments is more important than imaging. And it is very difficult to mimic the sound of a venue because of your room’s unique acoustics. Listen and choose. Happy shopping. 

I personally do not think of Marantz and audiophile quality.  I use their largest receiver in our family room hooked up to a Definitive Technology surround speakers and a much better system in the living room.  Perhaps you should stretch your budget and upgrade your front end at the same time.  Not sure what to recommend for speakers.  I added a pair of REL S2 SHO subwoofers to my system and they filled in the lower end seamlessly.  Their high level connection technology allows them to act more like woofers.