There are lots of good DACs available.
But first, the fact that your NAD contains an ESS Sabre DAC is not the determinant of the sound. It is the sum of the design and all the other components. So, while the really inexpensive DACs may have a similar sound, that is no indicator that other, better DACs will. The sound quality It is the result of every aspect of the DAC. .
So, for a budget DAC, I highly recommend a Schiit Yggdrasil or the less expensive Gungnir. These are great values for the money. I helped a friend get a Yggdrasil for his modest main system and in the process got a Gungnir for my small office system. Very detailed and natural sounding for the money.