Dimming lights isn't due purely to the wattage of an amp. It could be poor vehicle wiring, a low output alternator, an amp with very few/small internal capacitors, a worn alternator, loose accessory belts, inadequate wiring, an old/worn battery, etc...
I'm sure you get the idea. BTW, a few on this board with dimming problems have simply upgraded the (-) battery cable which connects the battery to the chassis and have solved/minimized their dimming problem.
