Thank you for the answer.
Unlike reported by other users on my end the exported MPDS in the AH 64 are actually sufficiently brighter than the exported MPDS in the F14 or F18. I would use Reshade to increase the brightness of the two latter planes.
My question in the first place was wether it should be basically possible to manipulate the exported MPD's brightness via the commands for the brightness knobs.
Increasing or decreasing the brightness of the TSD background map via the VID knob shows on all monitors, but turning the brightness knob only shows on the main monitor.
Is the only way to manipulate the brightness via changing reshade profiles on the fly?
Like it is recommended here:
https://forum.dcs.world/topic/299921-ah-64d-exported-display-images-look-very-dark/?do=findComment&comment=4960584