So here it begins, I put this into text.
Several years ago I acquired a 07' Impala LTZ through the family and it was a perfect vehicle for thousands of miles. I put a sub in it at around 75k miles and that didn't last long because the alternator failed and I took the sub out. I replaced the alternator with one from Autozone and it was fine. About a year later it seemed like the voltage regulator on the alternator was failing because the lights inside the cabin were dimming and brightening while driving. I tried to read the voltage but I could not replicate the scenario. I replaced that one with one from NAPA and that fixed the issue for a while. At around 85k miles I was driving and the voltage was dipping down to 12v and all the way up to 16v. Welp, there goes another, replaced it again with one from NAPA. I drove the car 1000 miles and parked it for 2 weeks. I started the car up one day and immediately the dash said charging system failed. It was not lying. Voltage control module was at 10.5 volts while running. At this point I was so fed up I just installed a junkyard one and that was yesterday.
All of this was over the course of 4 years.
I just replaced the battery because it was the only thing that changed when the alternators started breaking. In cold climate, they don't last long and it was from Walmart in 2013. So I hope that was it.
I'm not an expert on voltage regulation, but I know, for some applications, the battery is part of the control loop. So if it's wonky, that can throw the regulation out of whack, resulting in voltage that's out of tolerance, which could also damage the regulator.
Assuming there are no other issues, such as a wiring problem, I've got my fingers crossed that the new battery will fix it for you.
The only thing that concerns me is the impact of the sub woofer amp. Could some vestige of its installation still be affecting the alternator? I know it's far fetched, but is there an un-fused wire somewhere that is intermittently shorting to ground causing over-loads of the alternator? I doubt it, but those are the crazy thoughts I have when I hear of four alternators having failed.
Hopefully, the new battery does the trick for you.