there is no perfect sub that everyone agrees on.
with that in mind, you need a 4 ohm sub that will closely match the RMS wattage of the amp-- there are many to pick from, so read the reviews, listen the sub if possible, and then pick the one you feel is best..
capacitors are bandaids and snake oil, plain and simple. they cover up poor power, and ground issues that can be easily corrected by careful planning, and correct wiring.
for instance-- my own setup-- i run a 300 watt alpine, and a 600 watt sony amp--900 watts at full power. i have zero dimming issues even at full power because i took the time to setup the system properly...
i"ll explain the basic concept of capacitors as well, so you can also understand why they are sooo poor:
alternator: water pump
a water pump(alternator) will keep trying to pump water(voltage/current) even if you are not filling the bathtub-- at this time, it fills the reservoir(capacitor). but the reservoir will never fill beyond what the pump can provide-- meaning that it will never add anything to the water lines(the cap will not add voltage/current to power anything).
when you turn the bathtub on, and off rapidly(equivalent to the amp powering the sub-- and yes, the sub is getting power on/off rapidly), the reservoir releases all of its spare water(voltage) when the water pump couldn't provide what the bathtub needed-- now, the water pump needs to not only fill the reservoir, but also deal with you turning the faucet at the bathtub on and off-- so there is even MORE draw on the water pump..
so all you had to do was carefully match the correct sized water pump(alt.) to the amount of flow you needed at the bathtub, and you wouldn't need a reservoir...
this is the basic idea behind capacitors. if you look at a forum like diymobileaudio.com, they have a entire thread that delves into the electrical properties that shows that caps are more of a drain then a help to a audio system.
you could also search for posts by member oldspark-- he has covered this topic thoroughly here, and at the 12volt.com..
and one last tidbit: most audio spl competitors do not run caps-- they run multiple batteries-- most have proven, with their own setups, that they can get higher spl numbers by using multiple batteries then by using any caps at all..