If after doing all those upgrades, and you dim the lights, a 2-10 farad capacitor in parallel with the 14 vdc (typ located at the amp) will solve any remaining issues.
Another method to reduce peak current draw, would be to use drivers that are more efficient. On guitar bass speaker boxes, bigger + heavier permanent magnets are used. Then far less drive power required for a given SPL. Your typ Marshall guitar amp is a 100w head unit...and each speaker box contains 4 x 12" drivers. 2 x boxes can be driven from one amp.
That concept may or may not be feasible for a car audio setup.