Originally Posted by SuperD500

Finally found it out myself.
By removing the corresponding fuse, removed power from the radio which then is reset after powering on again. The volume (power) at the subs is back. Double checked by setting one of the User menu's to Linear again. What a difference! I wonder why this isn't in the manual: when set to Linear, you cannot get the original setting back.
I suspect this initial setting is factory tailored for the proper sound balance. And we might not even need this mod ...?