Major Voltage Drop

[SeRReAL]

The other day i was at work and was really curious on how bad my voltage drop really is, so i decided to hook up a volt meter to it and see how bad it really was, so first i tested the car resting i got 13.3 then i turned the car on with nothing on just the car running and got a 14.4 on the volt meter, then i tested it with the sub with the car on and then it read 12.4, and i was like thats not good so, then i tested it with all of the accessories on along with the sub, then it dropped down to 10.95 and i was like that is definately not good, so my question is this what kind of charging system should i go with, price wise is no problem for me, what i want to do is run a dual alternator setup with 2-3 batteries but i also want to save my ac, but also im planning to upgrade to a bigger amp, the amp being a memphis mojo 4000 watt amp, but i was also told that i could do a 300 amp alt with 3 batteries and have enough power to run the memphis mojo 4000 watt amp, but if i were to go with either a dual alt set up for a single alt setup like a 300 amp alt would i need to change out part of my stock electrical system or would i be fine with what i got, because i have heard of stories of people doing this upgrades and frying something so if you guys could help me out with my situation i would gladly appreciate it, and this is all going in a 2006 nissan sentra, my current setup right now is a power bass xa1500 mono block amp powering a memphis mojo 12in sub in a 4.5 cubic ft box, with a batcap 300 also with a trunk wall. So if you guys could help me out i would really appreciate it thanks.