I read a professional article about capacitors that said they've never seen any study that proved that adding a capacitor helps your stereo in any measurable way, and that if a capacitor truly did make a difference, every stereo manufacturer would be selling them and bragging about the measurable differences a capacitor makes. Since nobody does that, you can pretty much conclude that a capacitor doesn't actually make any audible difference. Their advice was to take the $200 or so that you were going to invest in the capacitor and put it towards a high output alternator, and a better battery and cabling.

I think that was pretty good advice.