Add a preamp or a whole new integrated

Looking to upgrade and specifically looking for better sounstage with chamber and orchestrsl music. I have a smallish room and a small system that does really does great on vocals and folk music. Seeking to improve how it sounds with Vaughan Williams, Elgar, and choral music. Needs more separation, better dynamics (?)

I have a NAD 352 amd and Cyrus 8x cd player with separate power unit. Hero interconnects. Tetra 120u monoitors (top ported, love em) and a velodyne subwoofer.

Would i be better off keeping the 352 and adding a preamp (NAD 165, parasound 210, cambridge 840e on the list) or ditching the whole thing and getting a better integrated. Budget to $1500.

An inexpensive preamp is not worth the trouble IME. I think your second choice is the better one. For example, you should be able to get a Moon integrated for your budget plus the resale on the 352.

And of course there are other choices; it seems to me that given your musical tastes you would enjoy tubes, as long as your speakers don't present wide impedance variations with frequency.
Agree with Tobias. You should also consider the Ayre AX-7E integrated - fully balanced w/ passive preamp stage and SET-like sonics. I swapped a NAD 352 for the Ayre a couple of years ago and it made all the difference for me.
Timbre, soundstage, and in particular, midbase resolution were dramatically improved.
Ayre AX-7E ... SET-like sonics
Sandstone - is your observation here based on first hand experience with SETs? Which SETs please? Thank you.