Thanks. I tried that, but it doesn't work really satisfactory if you want to toggle between "off" and "smart".
A few observations:
1. There is one function that enables "replay gain smart", but doesn't support toggle to also disable it, it simply activates it and it stays on.
2. When using "Toggle volume adjustment" (de: Wiedergabe Lautstärkeanpassung umschalten) I am toggling between all possible modes.
3. Once turned off, then it needs three more keypresses to turn "replay gain smart" back on.
4. Additionally it's easy to loose the overview, in which replay gain mode you are, especially when the replay gain values are low.
I would wish additionally for an indicator, which replay gain tag is in use, if enabled: intelligent | title | album.
Many thanks though for your kind suggestion.