problems with voltage on amp while settings gains

[creyc 10+ year member]

but does it really matter which way you reach the desired voltage?
For example:

Setting the headunit to 30/50 but turning on bass boosts on HU then setting amp VS leaving everything flat and turning the HU up to 40/50.

Each way it reaches 40 volts on the DMM.

That was just an example.

It does matter because going the bass-boost route can induce nasty distortion into the chain. A system is only as good as its weakest link, so how you get the right signal level is rather important.

To the OP, what is your head deck output voltage, do you know?