NAD DAC 1 seems like huge waste of money at 290 UK Pounds -
http://www.whathifi.com/nad/dac-1/review
and review isn't glowing ether.
I would use external DAC with pc only in two unique cases: I needed to get rid of VERY audible noises from PC - I am not talking about small stuff here and b) headphones needed beefer amp than pc output could provide.
In any other case most likely your exiting pc or mac internal DAC is just fine. Even worst realtek HD-Audio IF implemented correctly and with proper analog shielding - has pretty decent specs surely above -100db THD which is more than enough.
If my pc has really terrible (prone to RF noise) dac, even for mp3s external dac could help.
Now to OP's question:
If you don't get any noticeable RF interference from you macbook with connected speakers/headphones - I almost guarantee you - any external dac would be huge waste of money (With obvious exception of musicians, but that's whole another story).
It doesn't matter if DAC in NAD or other expensive amp is whole 5% better than one in you macbook - you won't be able to tell the difference with matched levels in blind test.
It's not the DAC fault your sound quality is not up to snuff - you need to look into improving speakers/sub