Unfortunately, most distortion/ovedrive pedals have some sort of level knob. You have to find a happy medium where the pedal sounds best without a drastic change in volume from when it's off. You also may want to get it out of your fx loop. Distortion pedals (my opinion) sound better outside of the loop.

Guitar -- distortion pedals -- amp -- fx loop send --chorus/delay/flanger,etc -- fx return = bliss.