Turn bass on stereo to 0 then if there is a bass setting on amp, maybe 1/4 or 1/2 way. Turn volume on radio up to highest listening setting, and slowly, carefully adjust gain.
Be sure gain is at 0 also before cranking volume. When you hear distortion turn gain back a slight turn. You need to really make sure you can not hear any distortion! Or else... POP!
Best way to set gain, o-scope. If avail by local shop do so.