Matching the gains appropriately actually does have a bit of a SQ benefit.

Part of the reason for setting gains is that every amplifier has an ideal voltage it wants to see. This voltage is some times hard to find from the manufacturer.

If the HU is not clipping (which is what we're trying to make sure of) you will have nothing but a clean signal, no matter how much you turn the gains up. You will notice more height, depth, and dimension as far as imagining and staging, not necessarily tonality, etc.

Stage two of this is as follows:

Mute the output of each pair of speakers all at once. Unmute and listen to the least efficient set of speakers (in our car it's the 5s) with a DB meter and get a reference setting on pink noise. Then mute those speakers and un-mute another set of speakers in the car. Adjust the gain on the amp then to make these speakers output at the same level as the most inefficient set of speakers. Do this for all speakers.

You will achieve maximum efficiency this way. With maximum efficiency, you will have more head room, which means more stage, etc. If the components in the amplifiers aren't working together, the components are all fighting each other. It's a symbiotic relationship.