My experience was that there was a noticeable, but subtle improvement in sound quality using an external DAC with the Node 2 (I didn't have the 2i). DACs used were the Auralic Vega, Mytek Liberty, and the DAC in my Sony TA-ZH1ES headphone amp (which is very good).
I obtained a much bigger increase in sound quality by replacing the Node 2 with a TEAC NT-505. It has a warmer, more detailed sound, and has a much bigger soundstage.
If you're going to add a $1500 DAC to the Node 2i, you'd get more bang for you buck buying the NT-505 in my opinion. It's about $2K and you can sell the Node 2i for $300-$400, so you might actually come out a few bucks ahead after selling the Node 2i.
I obtained a much bigger increase in sound quality by replacing the Node 2 with a TEAC NT-505. It has a warmer, more detailed sound, and has a much bigger soundstage.
If you're going to add a $1500 DAC to the Node 2i, you'd get more bang for you buck buying the NT-505 in my opinion. It's about $2K and you can sell the Node 2i for $300-$400, so you might actually come out a few bucks ahead after selling the Node 2i.