Hi, Philipp-
I admit I got a bit lost in the description about what the actual question was. Can you please rephrase it directly?
If you are wondering about a factor of 2 difference in a log-plot-based comparison, then I suspect one "power" method is actually calculating magnitude---that is, power is (magnitude)^2, and when you take the log of that, the 2 comes down on the lefthand side:
log(magnitude^2) = 2 log(magnitude).
You are migrating among many software, so it is good to make sure what is being calc'ed in each---first whether it is really power or magnitude (=amplitude), and second that log() is really the consistent base of logarithm you want.
--pt
