Voltage Dropping/ Electrical Issues

Sounds like a bad alt or reg going bad

That's interesting. When you start your car in the morning, do you have any problem? Maybe low voltage crank? After starting, you say it runs around 12-13 volts which sounds like straight battery power, and then drops after bass notes. Now, if it drops to let's say 11.4 volts and then jumps back up to the 12-13 range, the alternator is working, maybe not 100%, but working, because it is recharging the battery (its intended purpose). There are ways of testing both battery and alternator. Some mechanics have a machine that has two jumper cable looking wires clip to the + and - cables connecting to the battery. That machine can calculate your amperage output. If you have a 270 and its only kicking out 80, you know you have a problem. For the battery, again mechanics can perform a load test to see how the battery holds a charge. Now, if those two tests pass with flying colors, it could be a bad regulator. A voltage regulator is used to maintain a consistent voltage output. It shuts off the flow of energy to the car's battery once the voltage goes above a certain level. It prevents the battery from getting overcharged. That might be going bad.

To answer your other question, YES with a Zv4 18" it's always smart to run another battery to keep feeding it a constant power arounf the 14+ volts range.