EU DEM in GRS80 EVRS2000 with geoid EGG08 --> Make it comparable to Black Sea 1975 for Romania

I got contour lines for western Romania in vertical system Black Sea 1975 which is based on elevation data with 100 m resolution. Now I would like to compare that to the EU DEM with higher resolution. How can I make those two comparable? Is there a certain Z value that I can use to lower or elevate the EU DEM?

I want to compare two DEMs with two different geodetic systems or “reference sea levels”. And those two reference sea levels are “Black Sea 1975” and “GRS 80 EVRS2000 with geoid EGG08”. And my question is how I can make those DEMs comparable since they – as I assume – have a different 0. The 0 of “Black Sea 1975” is somewhere in the Black sea and I don’t know where the 0 of GRS80 is.

Here is a famous case about using different reference sea levels. I hope that makes my question very clear now..