need amp recommendations for more separation of instruments

@ahuvia My suggestion is to use a separate power amp while keeping NAD as a preamp. You can take the signal from pre-out on the back of the NAD and connect it to a separate power amp to see if you can get what you are looking for. In my system, there are a number of amps that put each instrument in its own acoustical space separate from others. Krell KST 100 is one and Audio research 100.2 is another. KST 100 is rated at 100w 2 8Ohms, 200W @ 4Ohms, and 400W @ 2Ohms. AR 100.2 has similar ratings except 360W @ 2ohms. Your speaker is fully capable of resolving this, but it needs a good amp that can handle its low impedance.My advise is to find an amp that can deliver enough current to drive your speaker at low impedance.

I have no way to know how your setup sounds compared to mine, but we have a few things in common. 
 

My setup is a Pathos Classic One Mkiii (similar price point to your NAD) and I have Focal 836v speakers. I also had the opportunity to audition my speakers on the Krell mono block gear that intended for the Focal Grande Utopia speakers. You could say that I “heard things” that I’ve never heard since. I actually selected the 836v over the 816v primary based on the sound of a harp in a Josh Groban song. 
 

I recently upgraded my source to a Denafrips Ares II and them moved to a Morrow Audio MA4 balanced interconnect and the combination of the two really improved the instrument separation for me.  My setup is far from optimized due to the physical limitations of a relatively small house, but I would consider a better source (depending on what you have) as a potential place to make a big improvement. 

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?

Yes.  And it’s free!  Again I ask, how far are your speakers out from the back wall?

@soix exactly right on all counts.

@ahuvia old mate from work had us over. I asked how he liked the speakers, and he mentioned they we better in the shop.
I asked if I could move them and pulled them out from the wall about 1/2 meter. (20” or maybe more).

Liking the sound, he said, ”How does the work?”.
I said, “like PF magik… Now just mark the floor with masking tape and move em around from here… It might get better.”

Of course he just left em there, but it really opened things up.

If we knew your room size it might help…

Mine speakers are about 3’ to a meter off the wall at one house.
The other place has more WAF constraints, and I may also need to change speakers..

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One of the best, if not the best amp I listened to since 30 years, what ever the price !