Would the NAD M51 be an upgrade to EE Dac plus

Or would it just be a different technology, sound
Lots of info on this Aussie forum where they compared the Nad to several well regarded dacs:

I have owned both. The M51 is better than the EE DAC Plus in every way. The M51 is simultaneously more relaxed and resolved -- effortless in its presentation, smooth, dynamic, and transparent. Is also stellar direct to amplifier.

For $2K list, it's a steal.
Are you using a stock EE DAC plus? If so, you may want to spend about $300 on some discrete opamps (Dexa or Burson) and some NOS tubes (Mullard, Amperex, etc..). To my ears these "mods" dramatically improved the sound for a rather modest investment. If you've done this and still don't like what you hear by all means look elsewhere.