Simple answer, an expensive DAC will improve it but it will hold it back. The internal DAC is pretty good. Against my Emotiva XDA DAC and my Rotel RC-1572 preamp's DAC it was better. Not much, but slightly more open sounding. When I hooked up it to my McIntosh C47's internal dac there was a major upgrade in sound quality, but it was still compressed sounding on the top end.
Like mrklas, I got a Moon 280d. It's a amazing DAC but didn't help the sound much. Got more detail over my C47's DAC, but the Node 2i was still compressed sounding. Now streaming with the 280d was no comparison. More dynamic, more detail, better imaging, bigger sound stage, and just about every aspect was a major upgrade. The only downside about the 280d is that you need to use the horrible app to stream tidal (no tidal connect) and even Spotify connect is rather buggy. Roon works perfectly fine on both platforms. Basically you're trading ease of use for far superior SQ.
If you're looking to drop 3-3.5k then grab the 280d and don't look back. The Lumin D2 is a bit cheaper, better app but is a streamer only with a bit warmer, less detailed sound. Equally as good but you lose the DAC.
