need amp recommendations for more separation of instruments

Thank you all. This is great advice. I'm looking forward to the adventure of tweaking the setup and I'm sure that will make things sound better.

But I want to double-check if I explained the problem properly. When I think of imaging, I think of the way different instruments seem to be "placed" on a stage between the speakers. That's not exactly what I'm worried about. My concern is that when, say, I'm listening to a symphony and the main theme is being played by the strings, but there is a flute in the background doing something a little bit different, the flute gets lost. I can hear that it is there, but I can't hear it clearly, it gets washed out by the louder string section. It is mostly a matter of the detail in the music getting lost when a lot of things are happening at the same time.  Is placement going to help with that?

Speaker placement and room acoustics is the first thing I would tackle. Equilateral triangle with speakers and listening position, then toe in to get the best compromise between image accuracy and soundstage width (it’s unfortunately always a compromise to some degree and requires time and effort to achieve a good balance).
 

Get some GIK or similar panels and treat the wall behind the speakers as well as first reflection points (read up on that…basically a mid point between listening position and speakers on the side walls). 

Few videos…

Re your flute question. Have you heard it live when it is playing in competition with a full string section? Unless it has been spot lit by a mic (many orchestras do use spot mics) I bet not. In a recording session the man at the mixer hitting the sliders will do much to accent the flute (or not). You may like the result or not. FWIW you may need a few great recordings which were made without spot lit violins (most are that's why the are often screechy and dominant and would easily hide a flute).

Don't blame your audio stuff for problems which may only be related to the way the recording was made.

Besides trying different speaker positions, consider having your room acoustically treated by a professional. Chances are, if the Focal dealer was using $100K of electronics to demo the speakers, I'd suspect they had also put a lot of effort in the the room's acoustic design.  That has a major impact on stereo imaging. As others have noted, early room reflections can smear the image of even the best speakers.

The only problem for many audiophiles is they do not have a dedicated listening room, but have their setup in a living room or other spot in the house that has to share duties with other family activities. While that typically limits what can be acoustically done to the room, that doesn't mean there can't be improvements.  That's where someone with experience in this area can be very useful.

My concern is that when, say, I'm listening to a symphony and the main theme is being played by the strings, but there is a flute in the background doing something a little bit different, the flute gets lost. I can hear that it is there, but I can't hear it clearly, it gets washed out by the louder string section. 
 

placement will definitely help but may not get you 100% of the way there. You will need electronics and cables that will bring it all to that level. DAC and integrated or separates. I’d say start with the integrated or separates. I’d say you’re looking at a $5k and up worth of upgrades to get you there. Might want to look at Hegel H390 integrated amp and something like a Chord Qutest DAC. At a minimum.