Which actual amplifiers are you running? if they are a 'true' 1500 W RMS each, then you absolutely have to uprate your power capabilities as they'd be drawing MASSIVE amounts of current. If they're '1500W' peak, then if they're a cheap budget brand, it's likely they're giving out perhaps 500-600 W RMS. You'd still need to uprate your power though as you will have other amplifiers for the mid range and treble with their own current requirements. If you have high quality brand amplifiers, then they'll have much more efficient and better internal power supplies and you won't really need to use a capacitor. If you look at competition level installs and excellent amateur level ones using high quality products, hardly any use a capacitor.
You need to plan your install carefully if you wish to run high RMS power outputs - have you considered all the 'true' power requirements, proper fusing and protection, correct grade wiring and the large amounts of sound deadening material you'll need? that's all on top of needing a very solid and strong enclosure for the subs, planning the rest of the system as well.