Hello,
I was in your situation (different gear), and found that adding a Bryston 4BST while using my mid-fi receiver as preamp was only a modest improvement (music had greater sense of ease, extension). Lived with that for a number of months until I saved the cash for a separate preamp, and that's when I heard the big improvement. My receiver was probably worse than your NAD, but in my case it definitely continued to limit the sound until I went with separates. Good luck.
I was in your situation (different gear), and found that adding a Bryston 4BST while using my mid-fi receiver as preamp was only a modest improvement (music had greater sense of ease, extension). Lived with that for a number of months until I saved the cash for a separate preamp, and that's when I heard the big improvement. My receiver was probably worse than your NAD, but in my case it definitely continued to limit the sound until I went with separates. Good luck.