OD pedals CAN be useful if the amps' overdrive has exaggerated frequencies or you're not able to play very loud. Here's something that I consider to be very useful, although it works best with an amp with a series loop: get something with an input and/or output level to go in the effects loop. Turn the amp up, and leave it where the volume gets the saturation that you like, then use the makeshift master volume to get the volume where you want it. If you can't get loud enough in this case, an attenuator used in conjunction will help. The amp's volume is the real key to getting good distortion, although OD pedals can be a part of the equation too.