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.