To figure Power values it is P=I*E where P = watts (Power), I is current and E is voltage. Your 800 Watt Amp will pull 66.6 Amps Most Headlamps were 55 Watts so that would be 4.5 Amps per headlamp thats 9 Amps for the pair. I would bet that your wiring and grounds through out the vehicle are pretty poor also.

Anytime you think you need to put in a Bigger Alt, also plan on upgrading the wiring also, at least from the ALT to the BAT connection.