Nothing wrong with a C.A.P.

But the "slash" amps are stiffly-regulated. They make the same power from anywhere from 11 to 16 volts. That plus the fact that you're 2 feet from the large battery, and I suspect that the cap doesn't help with SQ at all, as it could with loosely-regulated amps.

But it won't hurt anything, and might even help the HU to sound better when the bass hits (or not). I'm glad you didn't make a special purchase, though, since you had bought amps specifically designed to avoid the need for external capacitance.