How good is Blue Sound Node 2 in WiFi vs Ethernet connection vs external CD player?

I think the answer to your question will depend on quality of the CDP you use as a transport. If you are just trying to compare the differences in sound quality between a WiFi connection vs. a hardwired RJ45 with all else being equal, I don't think the differences will be significant assuming you have high speed broadband internet access and your router is not too far from your Node 2. Needless to say, hardwired connection is immune from occasional loss of reception between the streamer and the router. Curious to hear if others have noticed a difference between the two connection methods.

I find myself not understanding the OP.  The Node 2 has a built in DAC.  It's ok but nothing special.  If the Node is connected to your pre amp by the analog connects that is the DAC that you will hear.  If you connect it digitally to an an external DAC then the external DAC will be heard.  Which is better?  Devil is in the details.  If you have a stupendous external DAC, I would respectfully suggest using it.  If Said external DAC is a $39 Wal-Mart special, use the Node.  When in doubt, listen yourself and see which you prefer.
   Wired vs Wi Fi?  The manufacturer suggests wired.  That's what I use.  My wi Fi set up is somewhat dodgy, thanks to the incompetent folks at AT&T.

I find myself not understanding the OP.  The Node 2 has a built in DAC.  It's ok but nothing special.  If the Node is connected to your pre amp by the analog connects that is the DAC that you will hear.  If you connect it digitally to an an external DAC then the external DAC will be heard.  Which is better?  Devil is in the details.  If you have a stupendous external DAC, I would respectfully suggest using it.  If Said external DAC is a $39 Wal-Mart special, use the Node.  When in doubt, listen yourself and see which you prefer.
   Wired vs Wi Fi?  The manufacturer suggests wired.  That's what I use.  My wi Fi set up is somewhat dodgy, thanks to the incompetent folks at AT&T.

I find myself asking the same question regarding the Node, or any streamer for that matter. If you already have a decent DAC that you're satisfied with, why would a Node be better than an Apple TV or Airport Express, or any cheap streamer (Roku, etc).

Surprisingly, there's almost zero information out there that compares, say an Apple Airport Express to Node 2 purely on the basis of streaming capabilities.

I looked at ways to integrate Tidal/Roon into my system for some time after also looking at the Node 2 and others I ended up buying one of these Ausus  VivoPC VM42 (or one of the many versions they have) multiple USB ports an Optical out to go to your DAC (or the USB for that mater if you need DSD) I find its great as a stand alone music server I can plug my external hard drives or NAS into I  can stream from WIFI or a wired connection. I added a wireless keyboard/ track pad combo to act like a remote ( the pc's come with wired keyboard and mouse) and pluged it via the HDMI out to my TV (could use a really small monitor as well). all said it gives me the flexibility of a PC but since I'm using it as a dedicated streamer and NAS it sounds really good and is dead quiet. all told less the $500 Canadian.