Caps belong IN the amplifier, not in front of it!
A good power amp has plenty of reserve current available through the power supply for transients and peaks in the music. This is achieved through fairly large capacitors inside the power supply stage of the amp.
Now, putting an equivalent type device in front of the amp is just, well, unnecessary. If you think you *need* a capacitor, you're wrong, you need an upgrade to your electrical system. Or at least another battery, but this, like the cap isn't a good solution unless your electrical system is capable of supporting ANOTHER battery on top of your already massive current draw from your amps.
So don't waste your money on a cap. Do a big 3 upgrade, then a deep cycle batt, then if needed, high output alternator.