Battery setup not working, need some advise please

I have 2 Rockford 1500.1 BDCP'S at 1 ohm, 1 arc audio 300.4. I have the big 3, stock 120 amp alt, and new Die hard advance gold AGM 875 cca under the hood. I have some generic batt that is supposed to be equal to somewhere in between xs power D3400-D3100. The car audio shop swears by this batt and showed me another ride that it was in and says they have been putting that batt in car's and trucks with system upto 4000 watts RMS with zero problems.

The battery was working fine for 6 days and then the warning light came on saying "battery saver active" meaning that the car is not getting the proper amperage. Then a few hours later the lights starts to dim when the bass hits. I've been told some many thing by some many people I'm confused. Some have said go with an isolator, others have said go with a yellow top under the hood and 2 yellow top in the back. I've been told that the Die hard even though it's an AGM batt does not work like car audio batts. Any advice would be nice and sorry for the long post but I didn't want to miss anything. It's a 2012 Buick Regal

The ground is run directly from the front to the back and I have no idea what the name of the batt is, it's from one of those battery stores, like battery plus, battery tech type of stores

Well you may have a grounding issue here what is the battery exactly.

Got pics of that battery? I honestly wouldn't put 3k+ on a stock alt with 1 extra battery. Do you have a dmm to check voltage on the battery?

What you need is facts not opinions from people who don’t know anything to begin with. I will give you some info but you really need to learn a lot of stuff. There are three different types of batteries jell cell, flooded cell, and AGM absorbed glass matt. For best results do not mix types because each type charges differently (fact). What happens if mixed is that one battery charges to fast or to slow and maybe not enough. This can also put a strain on the alternator. How many batteries or how big of an alternator has nothing at all to do with some half wits opinion. You need to take a DMM and test your current draw to see what you need it’s that simple. Listening to people’s opinion is like guessing at what is needed and that can end up costing you a lot of money. If you get a good DMM you can test everything yourself and you will no longer have to be confused.

i bet you worked your alternator to hard and fried it. get your batteries tested and your alternator tested. it's free at most auto parts stores.