hi bernie,
having thought about it a bit more, if you go to the edit pages you should be able to have the expression pedal set to on/off for the upper 1,2,3 and the lower, which is independent of the sequencer.set your overall volume levels for each of these to say around 120, which gives a bit of "headroom" and then when you decrease the volume of the midi file you will still be able to control the volume level of your "live" sounds...sorry i cannot remember the exact tab presses to get there.
dennis
PS the above assumes you have an expression pedal!