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Does your truck have the positive wire terminal block that splits into two? One goes to starter and one for everything else? I've seen starters carrying a draw which can be quite high....so could try just removing the starter cable and see it the system will run. Few options to go over tonight lolAlso when I changed my alternator I removed all the wires from the big three to see if maybe one of them was causing the issue but things didn't change.
After doing some research I found out that AGM batteries can go into a deeply discharged state that will read low voltage. Also that a traditional battery charger won't charge a AGM, cause a AGM needs to be on a trickle charge for about 6-8 hours. So I'll be dropping off my battery at Autozone in a bit in hopes that it fixes some of the issue.
Are you running a big capacitor tied to the power side of the sub amp? If not, this will stop it from sucking all the power from the rest of the car (the capacitor will dump the needed power and save the stain on the rest of the system). My headlights use to dim on big bass hits - since the added cap no issues.
Also, check to be sure there is no ground loop. Ypu should have your amps grounded to the same bolt, if possible.