Should I buy a seperate dac?

Correction.
That NAD I had was the c 525bee which sold for $350. I still have in in my workshop

Generally speaking, an outboard DAC will improve your sound.   That's always been the case with me, at least.  Of course, there are exceptions.  That's why I said generally. 

Listen for yourself, is my honest suggestion. No reason to spend money on an alleged value that your ears don’t enjoy.

You may find a DAC that for you is just super delicious sounding. That’s when you buy.

In today's day and age though a streamer/DAC combo may be a better value for you, giving you an excellent DAC plus the ability to listen to music from Qobuz or Tidal and hopefully an excellent user experience.

Not sure if you can still find them, but the Parasound ZDac was an absolutely great sounding DAC for peanuts.   If you are dac-curious, a unit like that would not set you back too far.

@erik_squires & @wentz: looks like ebay has lots of them.  Mine has been going strong since they were first introduced.   Agree, great little DAC.