What do you mean by "All that without “the Big 3”?
Your 600w Kenwood door amp isn't RMS it's a max rating which is essentially a worthless #. It's likely about half of that in RMS.
Driving a 1200w RMS sub amp will likely dim your lights slightly when the bass hits and that's with or without another amp. Driving two amps totaling about 1500w RMS will dim the lights when you have it cranked up. I never found this to be a problem in my older cars but in a newer car with a sensitive PCM it may be idk.
You could and imo should install a capacitor on the 1200w RMS amp for the subs. You could then probably leave your stock alternator in there with no further worry.

 

I meant I have yet to do “the big 3” upgrade. Yeah the kenwood is a 75x4 channel rms. I plan to switch to the pbr400x4 after I get this installed. So the dimming will happen no matter what? Even with a better battery, capacitor and the big 3?Recommended capacitor? I can deal with it just curious. Side question. It’s a 5 speed manual if it dies can I just push start it? Will the capacitor protect the pcm?

 

As I said with your stock alternator and planned system without a capacitor(or wire upgrade) you will definitely get pulsating lights with a big amp and subs but with a cap you may not get that issue at all or if you do it will diminish it.
However, if you increase the ground and alternator wire size like you mention(big 3 upgrade) then that will be a better idea than a cap. For a 1500w system 2 gauge wire is better than 4 gauge. And really a higher output alternator would also be a better idea than a cap if the bigger wire size doesn't smooth things out.
75w RMS for a door speaker is more than enough especially for your speaker rating. If you drive 100w RMS into a 75w RMS rated speaker what do you think is going to happen? Rockford Fosgate has long been my preferred amp brand but that 400x4 amp is too much for what you have.
What better battery? Do you mean a bigger capacity battery or an extra battery? You won't be able to fit a much bigger battery in your engine compartment
A capacitor will help deliver more consistent power to your electrical system but it's not necessarily going to be great for audio fidelity. The quality of the capacitor will also be a big factor in smoothing out your pulsating accessory drains like from the headlights. The faster it recharges the better it will be. However, a capacitor taxes your alternator even more and actually steals from the amps after the bass hit and this is just one of the reasons why your audio may suffer with a capacitor.
A capacitor will just help smooth things out since it acts as a buffer but if you decharge your battery below 12v from driving the amps hard for a long time(hard to do really) it won't keep any potential glitches(in theory) from happening with the car's PCM.
I also don't think you would be able to jump start it with less than 12v in the battery but as I said you would have to be driving it hard and long. As long as the car was running I think the alternator would keep things at the minimum.

 

Yeah I meant an extra battery. Ideally I’d like 1 singular battery. I heard basically thats what a capacitor is. I’m looking for a good one but I’m not finding anything with good reviews. Ordered an r2-1200 x1 today from fosgate through Amazon. The pbr400x4 is too much? How exactly? I’m running this Kenwood 600w (max) I haven’t bought the 400 yet I just figured it would be a better option that the door amp I have right now. I’m definitely not trying to sacrifice audio quality. That’s what I’m paying for right good audio quality right lol. But the budget is a factor. I do plan on getting a powerbastard ho alt they have good reviews through this site. But for now I’m not trying to fry my pcm and I’m not wanting to sacrifice SQ is the whole extra battery the only way to go at this point?

 

If you were going to get a capacitor: capacitor at Crutchfield these T-Spec ones have good reviews.

 

As I said before if the speaker rating is less than the amp rating you will blow speakers. Your speakers are 75w RMS and if you drive 100w RMS into them you will be putting more power to the speaker than it is rated for which will blow the speakers at higher volume. Your Kenwood is 75wx4 RMS. You stated that your 6x8s are 75w RMS so it's a perfect match. If you want Rockford then get a 75wx4 amp from them.
Door speakers usually don't need such extreme wattage unless they have high power handling like some 6x9s do.
Just get the bigger alternator. Adding another battery isn't going to give you the benefit that the bigger alternator will and 2 batteries will be a bit of a pain in the ass to do.
Don't worry about your PCM. It'll be fine.

 

Yeah I get what you’re saying now. Yeah I don’t want to blow my speakers. I’m planning on going with the capacitor and a ho alternator…eventually. This is my first lil system so I’m sure I’ll learn like everyone else did lol.

 

Increasing the wire size of the 3 cables is one of the better ideas for you to do. The bigger amp alternator is also.

 

What gauge should I do the big 3 with? I’ve heard from an alternator rebuilder that I don’t want to go too low or it will make it idle low. Or be bad in some way I forgot the specifics

 

With a 1500w system at least 4 gauge, maybe 2 gauge.

 

Okay bet.

 

That's really heavier than needed for upgrading your existing wiring. I wouldn't put 1 or 1/0 gauge on your alternator as it's overkill.
You aren't buying a 200a or bigger alternator and your system doesn't exceed 1500w. 2 gauge is fine for your application for the big 3 upgrade and even 4 gauge is probably just fine. For your system 2 gauge would be erring on the heavier side. 1 or 1/0 would be a waste of $ with no real benefit. If you had a 2500w system, 200a or more alternator, or 15ft or more wire length then ok but otherwise there's really no point.
These wires are simply supplemental to the factory wiring unless you plan on having someone chop up your factory harnesses. I do NOT recommend that. They get heavy ring terminals that go under the factory ring terminals and the new heavier wire basically runs along the factory wires/harness. Typically, they get zip tied to them. Additionally, the new alternator wire should get fused as well. Having these wires in addition to your factory wiring gives you a backup if something happens to one of the new heavy wires. You would still have the factory charging system still working.