It's not just about power. Of course I don't know how good your NAD is. I start with a Denon receiver, then upgrade to a Rotel receiver, than go for a Adcom preamp and Rotel power amp. The improve going pre-post set up is night and day. Then I upgrade my Rotel power amp to McCormack DNA-1, almost no difference. Then I went through a couple preamp, each one give me a different and better sound. I don't know the technical reason behind, I am only sharing my experience. Going from integrated to pre-post gives me the biggest improvement in sound quality even with a cheapy Adcom preamp. And changing preamp has bigger impact than changing power amp.
And your question is similar to using a CD player output directly to the NAD, versus adding a DAC in between. I use a cheapy Sony Blu-ray player, adding a DAC, in my experience, has only subtle improvement. Of course, YMMV.