Capacitor?

[userfriendly]

theoretically, pwede ka bang mag kabit ng capacitor kahit na wala kang sub?

Yes you can but your wasting money imo. An analogy for a capacitor is the water tank in your toilet. The tank holds water for use. The capacitor holds an electric charge for use. With a stock system or a system that does not have a huge electical drain the battery and alternator can supply it. Same with the toilet, say your flapper valve does not close, you won't run out of water. The tank is there for flushing or in the case of the capacitor, its there to provide for the sudden electrical surges that your sub and amps will need.

Normally, you won't need a capacitor unless you see your headlights dimming when the bass hits. Even then, I'd suggest upgrading your ground wire 1st.