And yet.. I run an amp of much more power w/o the big 3 and without issues on a 125amp alt.

As the cars age, ground and power connections become corroded etc. The side terminal connections from the battery are notorious for corrosion. I have fixed a P0171 Lean Bank One by replacing battery cables on a car w/o any sound equipment modifications. Things like this are why I suggested checking those connections. Checking is totally free, takes a few minutes and a set of eyes.