Its sounds like you're using a 5 ch amp for all your speakers. I did a quick search is that the Rockville RXH-F5 3200 watt peak/1600 watt RMS amp? at 14.4 (running) that amp alone is pulling 111 amps at your 1600 watt rms... When you crank it up, double that (222 amps) , and another 20% if you are running it with the engine off at 12v..
YOU NEED TO UPGRADE YOUR ELECTRICAL SYSTEM...
You're vehicle is normally using roughly 60-80 amps depending on if you are using fan, ac, headlights, etc... and about 110-120 amps if you are running everything and charging phones, fogs, FI, brake/turn/cruise etc....
Your wiring is not big enough to allow enough current to flow. One thing I've learned about upgrading a stereo, is plan for the end game... Figure out how big you might possibly want to go, figure out your numbers: how many amps you will need to count for and what size wire you will need. Do your big three with that wire or bigger (your big 3 is running your stereo and the rest of your truck)... as long as you use the appropriate size fuses at the lead of your wire lengths, you can not go to big on your wiring... if 1/0 wire work so will 2/0 welding cable... Its going to be a kick in the B-hind to upgrade all your wire again for a second sub (money and time) when you knew you wanted to do it in the first place, but its only a few bucks more (relatively) to do it the first time.
CAPS ARE BS (for 99%). Any properly setup system is not going to need them.... A cap is just a temporary storage.. Think of taking a shower through a straw. A cap is like a cup in allowing you to store up the water when you don't need it so you can dump in when its time to rinse off/for the subs to hit... Upgrading your wiring is like upgrading to a garden hose, or even better a fire hose, and then you can run it below max..... Some systems can benefit from them, its a cheap way to band aid a cheap setup, or a way to make a large setup slightly more efficient.
You NEED a better alternator... The gen III uses 2 sizes of alternators for all its engines (4.8-8.1 & 6.6). A 105 amp small case design and a 130 amp larger case design. All alts that fit are based of one or the other. The larger case has a slightly larger pulley and uses a belt that is 3/4" longer. They bolt up the same, you can use either alternator design with the correct belt, (and there are 2 or 4 plugs on the alt).. I would stay far away from the alternators on ebay. Do your research if you plan to get one, most of those companies sell junkyard rebuilds. I bought one of those and it whined, and then I learned about "output at RPM". What they also aren't upfront about is what the output is at idle and what rpm is max output. Its best if they can show you a graph showing the output curve (they should have one). I've seen at least one alternator off ebay that didn't give its stated/max output until 3200 rpm (I swear it was 300amps and 80amps at 700rpm which is worse than a factory alt)..... Who cruises at 3200 rpm???? Do you plan on turning off your stereo when you are at a light???
I spent $5 or $600 on a mechman 300 amp alt and its great. IIRC It has 220 amps at 700rpm and max output 1200rpm. As with most all automotive aftermarket, You are going to pay twice as much for a 10% better product... Quality here is a lot better than 10% better.
