What's unusual about the Codex is the lack of digital coax input and you need to use the optical port which is generally less desirable, at least based on what I've read. I'm assuming that's how the Node 2 was connected to the Codex since as far as I know Node 2 does not have a USB output port. As for the OP's question, I have used a Cambridge Audio DACmagic Plus with Node 2 using the digital coax output port and I didn't hear much improvement. I now just use this DAC with my CDP which does make the CDs sound a lot better.
Experience with Bluesound Node 2 use with external DAC
I am very impressed with the Node 2. So far my use of the Node has been with it's built in DAC. It sounds great. Since I don't own an outboard DAC I have had no other choice but to use the Node's DAC. It is hard to fault it's DAC at any price much less the $500 price of the Node. That being said, I realize there are alternatives out there than can improve upon the built in DAC. I am at a point now where I am curious to hear what an external DAC may have to offer in terms of sound quality. For those of you who are running a Node through an external DAC, what would you recommend and what type of improvement have you experienced vs DAC in the Node.