How to get 15v

[Bolt cutter]

As I am waiting on my new radio, I am taking the time to make sure things are correct with the set up.
I was checking on how many volts I had at a connection under the driver seat where I have my amps for the stereo. I showed 14.26 volts. Checked at the battery and got 14.27 volts.
If I need 15 volts to run a Dave Made 1X2 at 400 watts I need 15 volts. Your thoughts please.

Bolt Cutter

[T2W]

you will need some kind of seperate charging system.

even if you could jack your alternator up that high you will cook the battery.
a 12v battery is not going to work very long at 15v

you should just run it at what voltage you have.
nobody is going to know the difference.