voltage drop on sub amp

I've been noticing a voltage drop on my subwoofer amp,
and I can't figure out the problem.

First, here is my setup:
1 phoenix gold 4-ch amp @ 300W RMS powering 4 speakers (4 ohms each)
1 Alpine MRD-M501 @ 500W RMS powering 2 JL audio 12" W3v3 (2 ohm load)

4G wire from batt to distro block, 8G for amp power and grounds.
The run of 4g to the block is about 12-13 feet I think.
The rest are 3-5 feet each.
I've also got 0 gauge Tsunami for the "big 3".

The voltage display on the alpine holds steady at 13.5 - 13.6 at lower volumes, but will drop to 12.5 or lower during heavy bass hits.
When this happens, the display will shut off for a second. I don't notice the subs cutting out or anything though, maybe just leveling off at a certain volume.

I was convinced the cheap 4g I had originally installed was the problem, so I tried replacing the power wires from batt to block and from the block to the sub amp with Knukoncepts KCA 4G, but the drop remained. I took the small breaker box out, and temporarily connected the + wire directly to the batt to see if the connections were the problem, but it still drops.

I have a multimeter and I was reading a drop of 25-30 Mv? between the Batt terminal and the beginning of the power wire. Is this acceptable?

I'm wondering if my setup is just too powerful for my 100 Amp alternator, or if I would benefit from a cap at all.

Any suggestions will be greatly appreciated,
and I will gladly conduct a test or three!

BTW, I lowered my gains recently to +0 for the alpine
and from about 75% to 50% of max gain on the Phoenix Gold. This greatly reduced the dimming of my lights with heavy bass, and I can get higher volume with clean sound from my speakers.
The interior lights still dim slightly with the bass, though.

Possible candidates are, bad connection at battery, fuse holder, or the distribution block. Bad connection on the ground path.

Ditto.

At full volume, 800W => ~80A which a 100A alternator should handle depending on RPM and other loads, but that's where you need to measure the battery (else alternator) output voltage and compare to amp voltage to see if it's a wiring voltage drop or an alternator voltage dip.

Forget a cap, you'd be better off with a (small) battery instead.

The subwoofers you are using...are they 2 ohms each or 2 ohm final load (4 ohms each)?

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I can not explain to you what "nothing" is in a scientific way. What makes matters worse is you won't notice it when you finally have the opportunity to experience it.