Volume setup:
Preamp is set to +-0db (if rginfo available), otherwise -6db.
Volume ist at max (-0db).

In case I plug in my headphones instead of equipment with two boxes and amplifier/vo. control etc. It occured with some more calm files volume is too quit, but being at max already (internally), it can't be adjusted to +x db.

My idea is that if volume is at max preamp values are raised both equally in order to raise overall volume over current limit.

Raising preamp noticably will cause most music to distort.

A more interesting question is why your headphones are so quiet with maxed out volume. What pair and what sound card are you using?

Cyfer (see here) also proposes a (albeit similar) solution.

The reason why my headphones are quit is that foobars internal volume is at max but windows mixer isn't. Sorry if I wasn't clear in my first post.
If I'm using the two speakers windows mixer remains unchanged when switching output, and if the speaker's own amplifier wasn't adjusted, music would truly be too quiet, but since it isn't so I use windows mixer to control volume because amplifier's controller isn't at all reachable from my computer.

If the approach of adjusting preamp in order to solve my problem by keyboard shortcuts wouldn't work well instead a alternative is what cyfer suggests (if I understand him correctly) or a new keyboard shortcut (inactivated) to raise master volume solely, or what I suggested in my OP, everything I said besides one needs to replace "preamp" with "master volume".
More clearly, if foobars internal volume is at 0db and user wants to raise volume level even more, foobar adjusts master volume level of all applications, all using one key for convience.

I guess we want the same thing. Somekind of function that can change foobars internal volume as well as the master volume (the pc's/windows output volume) at the same time.