Voltage dropping to 11.8v at the T1500bdCP terminal

do you have a volt drop reading from the battery post to the amp terminal while the engine is running and bass is hitting? if your chassis ground is corroded it acts like a resistor and heats up under heavy current which explains the cutouts and that annoying whine creeping into the speakers. i had a similar situation with my skar setup where the ring terminal looked fine but had invisible oxidation under the crimp and it caused my voltage to tank whenever the vxf 15s really moved. if the voltage at the alternator output post stays steady while the amp input drops then your ground path or main fuse connection is definitely the culprit rather than the alt dying.

I had this happen with my 2x NXL2000 setup when my main fuse holder started melting internally. If you have a glass AGU or a cheap ANL fuse holder, the set screws loosen up over time and create heat that drops voltage way more than a weak ground. You basically have two choices: pull your entire fuse block apart to inspect for melted plastic or burnt terminals, or just bypass it temporarily with a fresh chunk of 1/0 OFC to see if the voltage holds steady under the same 30hz load. I would swap to a high-quality bolt-down fuse block like a SMD or Shuriken one because it locks the wire tight and keeps resistance low. Does the volt drop happen instantly when the bass hits or does it slowly sag over a few seconds of heavy notes? I keep wondering if my own DS18s are pulling too hard for my current wiring too.