You still haven't answered what kind of electrical upgrades you have.
You do realize that 6000 watts rms is over 1200 ampere's of current draw at full tilt, I don't see anyone with a daily set-up that is capable of sustaining that much amperage demand.
Secondly your not taking into account, a thing called thermal compression. Once the coils get to a certain temperature they absorb less current, you are loosing efficiency. You can break 150dB with a great box build and 4kw.
If this is for daily, even a XXX would have issues dispersing all of that heat, and it would 'possibly' even have mechanical issues with 6kw @ 28 hz.
Just to give you an idea of the cost to run a daily 6kw set-up. You are looking at 5-6 runs of 1/0. 4 group 31 batteries, and at least a triple alternator setup. And that is for a bare minimum. You probably still wouldn't be able to full-tilt your system for more than 30 seconds with the car on.