You can run the 850 watt amp to your sub and just turn the gain down. It's always better to have wattage "headroom" rather than not having enough wattage for any speaker.
To set the gain properly:
-turn the sub's gain all the way down
-turn up your inside speakers as loud as they can play without distorting
-then, with your inside speakers at full blast without distortion, slowly turn the gain up on the sub amp (with your head right next to the sub so you can hear it over the other speakers) until you hear distortion.
-when you hear the sub distorting, turn the gain back down a little bit until it sounds clean again
When your setting the gain, I recommend using a cd that plays loud and clear, you know, a high quality recording with real clean sound, and make sure your sub level on the deck (if you have it) or your bass knob is all the way UP before you go through this process of setting the gain. Otherwise, if you inside speakes are playing full volume, and THEN you turn up the sub level all the way, it's gonna blow.
That sub's going to be getting all the power it wants and will be happy as a clam.
