The OA3 in the TORII feeds high voltage the the screen of the output tube, which determines the overall gain of the output tube and ultimately the resulting power of the output tube.  By changing to a OD3, you lowered the voltage to the screen and dropped the gain of the output stage and subsequent power.  However because you did not effect the voltage or gain the of the input stage you would not notice any change in performance until you reach max output.   The bass tightens up because the gain of the output stage dropped slightly.