The problem lies with the purpose most people believe capacitors are for. Are they magical and make power? Of course not and neither do batteries. The proper application is not some magic band-aid that helps when you can't supply enough current. It's to prevent voltage dips. If the voltage is going to dip and stay there, the cap won't help for long. If the voltage is going to dip and come back up as with most transient music, then a cap can and will help. Caps discharge and recharge instantly. They won't wait to do it. That's why there is such a thing as "inrush current". If caps waited to recharge, how in the hell would a capacitor in a computer power supply work? Computers draw a very consistent amount of current all the time. If they just depleted and never recharged how would they filter the 60hz ripple?
So in short the proper application for a capacitor in the car audio world is after the installer has provided enough current at 14.4v via the alternator so that the amplifier can play at full tilt and the system still maintain very near 14.4v. Pick several capacitors with the lowest ESR you can find and put them as close to the amplifier as you can. They will filter the ripple and keep the sound clean. Again, the reason these idiots think caps have no purpose is because they don't understand the purpose of them.