Would this amp and cap be good or bad for these subs? as far as having the right amount of power to the subs?

I would need to know the model of the subwoofer to know if the rms ratings are bloated or not, but regardless I would not recommend the volfenhag amplifier, not only because of the brand itself but also because of the power. Your subs have single 4 ohm voice coils, that means they can be wired either to two 4 ohm loads on a 2 channel amp, or a single 2 ohm load on a mono amp. Your best bet would be a nice mono amp that produces about 800 watts at a 2 ohm load, that is if your subs have a true 400 watt rms rating. As for a capacitor I would say it is not necessary, but if you really want one you would only need one farad for the current draw of the amp you need.