If I had a 15" Icon and a 1000 watt amp would I need to get a new battery under the hood, do the big 3 or get a new alternator in a 2004+ Explorer or a 2001+ Tahoe? What about 1500 watts?

All of those won't hurt you, but at the very least you should do the Big 3. And then move to a larger battery.

what amp are you looking at ?

biggest batt you can fit up front and big three should do you very well

what amp are you looking at ?

I got the kicker zx750.1 since its underrated & i got it for under $200 but i want to get a more powerful amp for around $200. I also chose this amp because i didnt want to get a ho alternator or get another battery but i might have to do the battery. Will the icons sound the same or significantly louder if i had 750 watts to it vs 1000??

if they do get a lot louder one amp is well under 750 watts or the 1000 watt amp is well over 1000 watts.