Worth upgrading from Rotel RC-1572 to the new RC-1572MKII ?

Without doing a deep dive on what they upgraded on the 1572, I would be surprised if you noticed an huge difference. If you want to stay with Rotel, I would go for the 1590. Preamps have a huge impact on your system, you might look around and see what else is out there in your price range. Consider a used solid state preamp, and for the same price you might be able to bump your system up to a new level. 

Are you using the DAC in the Rotel or the one in the Nano? What type of cable are you using to connect? In either case, the DAC’s in these units are afterthoughts, an upgrade there might be worth considering. 

The Nano is a $300 unit?  I can only the imagine the Texas Instruments DAC in the 1590 is orders of magnitude better.  With a nice digital cable you could see some real SQ improvement in streaming. According to the review I glanced at some of the design and less costly parts have trickled down from the Michi line as well.  1590 has dual transformers I believe which would also be a nice bump. 

The Node Nano is the latest offering from Blusound. It carries the new ESS SABRE ES9039Q2M DAC.  I would hardly consider that an after thought. My Rotel RC-1572 carries the AKM DAC. The new MKII version has the TI DAC

I'm using Audioquest Black Beauty RCA's between the Node Nano and the Pre amp. And between the Pre Amp and Amp

My Rotel RC-1572 carries the AKM DAC. The new MKII version has the TI DAC

Its the implementation of the DACs capabilities that really matters. You would think a DAC in a pre amp that cost several times as much as the entire Nano to have put the work into maximizing capabilities. Have you compared the SQ between the Nano DAC and your existing Rotel?