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.
Would the NAD M51 be an upgrade to EE Dac plus
Or would it just be a different technology, sound