a higher output alt would be a good way to go, from how i have had it explained the alt supplies the power to the batt regardless so the amp gets most of the power from the alt, getting a 2nd battery would just place more load on your alt to charge two batteries instead of one, getting a stronger batt is good but on the long run a stronger alt is nicer....

contact dominick@tds.com, he makes h.o. alts for anycar and can usually make them to fit in like the stock alt