DAC? how much more $$$ to get a significant improvement over Bluesound Node?

BRYSTON……you won’t be disappointed and highly likely be surprised.

because they are high-end units and built like tanks, the BDA-2 is a steal at a pre-owned price. 

https://www.soundstagehifi.com/index.php/equipment-reviews/597-bryston-bda-2-digital-to-analog-converter

Any of the better $800-$1K DACs would be a significant upgrade of the Node's DAC. If you go to ~$1800-$2500 range, you're in a different level for many of the manufacturers. There are tons of threads even just this month of people describing the pros/cons of each. 

To help narrow down your search, you might want to group in your mind different types like R2R ladder DACs vs. chip DACs, and based on your preference narrow further from there. 

For example, on my recent hunt to upgrade from my Schitt Gungnir Mulitbit, I first decided I wanted another R2R DAC or an FGPA DAC and not a DAC running an ESS chip or similar. From there, in price range I researched and narrowed down to the Denafrips Pontus II and the Chord Cutest. Then based on my listening to other Denafrips DACs, descriptive comparisons and taking into account connectivity, features, etc. I chose the Pontus. There's plenty of good ones out there, and it can be a little overwhelming if you don't break down the problem into manageable pieces. Cheers,

Spencer

$300 + could be better.

+1 for the Pontus II!

My guess is $500 will start to show an audible improvement.  Remember the Node cost around that for both a streamer/DAC.  I have heard the Node paired with a CA DAC Magic, which is long in the tooth for a DAC and goes for around $500, and with the Cambridge there was a notable boost.