i would suggest a rebuilt alternator for high current amps. How many amps does the amp draw? how much amerage doed the alternator put out? if ur alt puts out about 60 amps and your amp needs 90... time for a rebuild!
rebuilt alts should cost about 150, and is much more effective. However, increasing the amperage output tends to make you lose HP.

one more thing, is that amp a class d amp?