Ash, what size cabling do you have between the battery and the amps? If the cable is substantial, you might want to look at a stiffening capacitor. This will make sure there is plenty of juice for those bass hits by storing energy where it can be released very quickly. Capacitors have low internal resistance, unlike lead-acid batteries.
I doubbt a larger battery will help in this case, although a larger battery will give you a longer engine-off listening time. Last option would be higher output alternator, as I think KA24 alternators are 80A, which should be plenty.