I have 2 Crossfires VR1000D and 1 VR404.
I had the same problem you are having now, with a voltage meter on the battery i would go as low as 12.0 and the amp would cut. I had all amps on 4 gauge wiring all around. i was fed up with it and ran 2 separate runs of 1/0 for each amp and also switched to 1/0 grounds for the VR1000D's only. I hooked up all the connections checked voltage on my alt, battery, and amp and they all read 14.2, so i blasted the music and it wold dip to about 13.2(While music is loud) and 12.9 on the real low sustained bass lines. I have a 04 Tahoe with stock 120 amp alternator. Try jumping over to 1/0 power and grounds on your big amp. It made a difference in my truck. Now i just have to finish the Big 3.
Good Luck!