When you deal with adding electronics to your car you are putting a strain on the oem electrical system..... usually a rule of thumb is that once you start talking over 1000 watts upgrades need to be made to your system( battery, alternator, caps)...... but what will happen if don't upgrade your amps can not get that power from your current battery and alternator and it will try to take it from other places like your headlights flickering and other strange stuff with cars electrical.... so when you say you have 2 1500 watt amps that is in perfect world with good electrical 14v, but in your situation during demands from you amps you may be sitting at 10 volts if don't upgrade possibly send your amps into protection mode(low voltage)