Good Entry-Level DAC

there a reasonably priced ( < $1K) DAC that could significantly improve the sound?

Doubtful. 

I still doubt any of these suggestions will significantly improve upon the DAC in the NAD. If you simply want a new DAC that’s a different thing altogether.

It's like you said in your original post it is a pretty good DAC in the NAD. It's not that some of these DACs mentioned wouldn't sound different, not necessarily better, but you were asking about significant improvement. You already noticed how you couldn't tell any difference when you got the NAD from it and the other DAC. If some of the ones mentioned look intriguing I would make sure I could return them with a trial period. I've certainly been there and done that. 

DACs don't extract information off CDs or Streams they convert what's been sent to them. The speakers, room placement and treatments will affect the sound a lot more than the DAC. The OP could also try a newer NAD CD player with the newer 24 bit Wolfson DAC cheaper than some of these outboard DACs. I still haven't seen anything here that would be a SIGNIFICANT  improvement over what he has. Especially if he auditions them in a blind setting. Another thing he has never said what the rest of his system consists of, if it's consumerate with the NAD I wouldn't expect changing a DAC unless it's really crap to make much of a difference.

Yes I did, sorry about that. There are dealers who take new returns. I think The Music Room has a return policy on used. 

Yes, DACs have an analog output and the noise floor can be measured on those outputs. No matter how much noise comes in on the digital side if the DAC measures noise at inaudible levels on the analog side then it’s doing job and is a well designed DAC. That’s why reclockers and supposedly noise lowering cables are irrelevant for well designed DACs. Say I use a lousy cable with no shielding and the noisiest computer I can find to feed a DAC and the noise is ridiculous coming into the digital side yet when it leaves the analog outputs it measures - 120db and is inaudible to humans and then use the best cables I can find and clean the digital before entering the DAC yet the analog side measures -121db was all that cost worth -1 db that’s completely inaudible anyway? It’s not 1980 anymore even a well designed $250 DACs can handle a lot of noise.