^^ There have been a lot of instances of people draining the 12v battery very quickly. By most accounts, when this happens, letting car sit for a while seems to shunt power from the IMA battery pack to recharge the 12V battery (about 15 minutes). Understanding this, if I was planning to put additional load on the 12v system, I might consider fitting a higher capacity battery - just see how large the battery tray is before you start shopping. I'd imagine that this 'need' would depend on how much power the new sub is drawing and whether you tend to play it while parked.