If you're using a powered amp, it'll pull a decent amount of current. The stock alternator isn't up to much more demand than what the factory accessories require to run. A friend of mine ran 2 big amps and his alternator would get so badly loaded up it dropped engine rpm (and it got pretty hot too)
btw it is bad a powered amp to run on low current. It can damage it plus it will affect audio quality. A higher output alternator and bigger CCA rated battery are worthwhile upgrades.