need amp recommendations for more separation of instruments

Speaker placement is critical for great performance as brought up above. If you have not done a lot of this… then think of placement taking hours of work over weeks. Not just move a few inches… listen, move, listen… done.

You might look up speaker placement you will find more than you want.

In short… you need space behind speakers, and away from walls, in equilateral triangle with listening position. Starting with them pointing behind your head. Move in and out left and right to open soundstage but not loose central image. Then optimize toe in. Less toe in opens the soundstage and separates images. Then get everything out of in between speakers and dampen the wall. This will unconfuse the images. If speakers too close to side wall, then dampening needed there… this goes on.

Anyway, I would recommend doing moves over very long sessions, over many sessionsm. Get to know the sound really well… then move… get to know the sound really well.

If all this fails. Then it will unlikely be tha amp as such… it will be the combination of source / preamp in your NAD. They all make a difference… but the amp would be the last place I would look. If you need improvements. I would consider trading in for separates. Too many functions in a single box make enormous compromises in sound quality.

My,speakers are about 9 ft apart, ….15” from the,wall, my sitting position is 9 feet from the speakers,slightly toed in. 
 

experiment, took me weeks to find the “spot”

once you find it, you will know.

also, I went from K-mart rcas to audioquest and Groneberg power cords , and still use my BJ rcas in main system, they cancel out the radio tower tweeter gibberish, broadcast.

experiment, move them, sit for 15 min,move,again, keep,a,written note of position or distance from wall, from listening position, all the goodie.  
 

take your time, as my wife said I was a crazed metalhead moron, moving, listening, moving, swapping,cables, different,cd,players, every day,for,about,a,week and a half.  You,will find your “spot” keep messing,w your system. 
 

I like moving stuff,around,as I,clean dust, etc from back of rack, also,when the stereo is,clean, t does sound better.

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.