With the AV10, you likely have the best measuring DAC implementation money can buy, especially when considering the exhaustive feature set and channel count that processor has. I don't know that the NAD M66 has been measured, but the company's reputation in terms of SINAD is not stellar and your Node measures particularly poorly. Now, that's not to say that this is remotely audible and those who have done blind A/B against a Node X have reported no discernible difference.
It's important to note that the M66, nor any other of the NAD integrateds supported a Home Theatre Bypass option for integration with a Home Theatre as you are asking for. This has been a pretty major sticking point with customers as most of their competition, be it Hegel, or Rotel, or what have you, all support HT Bypass. I suspect they don't implement it because BluOS adds quite a bit of processing delay into the signal chain (at least 50 ms).
My advice is to forget about an external DAC. It's a waste of time. It's certainly worthwhile investing in Dirac Live + DLBC if you haven't already. If you're using the Node for Roon, which it is great for, then carry on. There have been rumors of the AV10 getting Roon Ready support, but nothing has materialized as of yet.
