Do you mean the Alpine MRP-500 outboard power amplifier? If so, look on the end which has the (red & white) RCA inputs, and look for a "gain" knob; you'll probably need a tiny screwdriver to adjust it. Turning it counter-clockwise will turn the gain down, turning it clockwise turns the gain up.
If this is not the amp/device you're talking about, get us a pic?
EDIT- just a thought; if you have an MRP500 you probably have an aftermarket sub too. If so, if there any polyfill inside the enclosure? You can get polyfill at any fabric store for under $10; lining the sides on an enclosure with it helps eliminate boominess.
