Not for the sole reason that a sub amp will take more power. In the ways a cap works, it helps fill in those power spikes created when a strong note hits. Everytime one of those notes hit, extra current is needed and the alternator is not always there to supply it. That is why you will see dash lights blink, it is because the current going to the dash lights is reduced, and is used by the amp. The amp you are using to power the in cabin speakers do not require a power surge or spike.