I though you wanted to just adjust the LH piano notes ?

but as mentioned above
"In the piano roll you can right click > Select All Notes, or highlight the Base notes for just Base, use Ctrl key and click on the Base notes on the side keyboard, now hold the shift key down and just drag one of the Velocity levels up"

If you want to turn the Velocity up on all the notes "In the piano roll you can right click > Select All Notes, then hold the shift key down and just drag one of the Velocity levels up, the little vertical lines at the bottom (have the View/Edit set on Velocity to show them) but will will need to do this again after re-gen.