A cap will help for a 1/1000000th second bass-line. After that the cap are useless since they discharge ALL of their stored energy in a nanosecond. If you had a couple hundred Farads of storage, it might help for a second or so. But all that energy would take a while to store up, rendering it useless. I would just recommend getting a battery first and then a high powered alternator if you are serious about audio.

In order of best to leasy helpful:
1. HO Alternator
2. High Current battery (Yellow top Optima). The rating on batteries gets difficult and I won't get into the discussion on plate thickness, CCA's, RC's, etc.
3. Cap (mostly worthless)