keeping the impedance match close is what matters. a lot has been posted here recently so you can hunt/search and read

 

Can someone please tell me everything I will need to add 2 12 inch subs and a new 1000 watt amp in the back. I want to keep the factory 6 disc head unit and the monsoon amp and sub. I just want to add an additional amp and subs. Will I need the AOEM-GM24? I have a 2006 Luxury Edition

 

Call Best Buy or any retailer of your choice that installs stereos and they will tell you what you need.

 

Is there any chance of removing the interior panels from either side of the cargo area and fitting an amp behind there?

 

2 12" subs!? your hummer is going to be one big rattle lol. this hummer is so bad when it comes to bass. you better remove all panels and install that stuff that makes everything dense and heavy and stops the rattle or its going to sound horrible with the rattles. good luck man. i belive the stuff you want is called dynamat

 

Its called sound deadner/dampening, Dynamat is a brand. There is better products out there for less money (Second Skin, Cascade Audio, Sound Deadner Showdown, etc). If you can hear the rattles, then its not loud enough