Charles,
If you have an Alembic DS5R in your rack to power your Series I , the front and back audio outputs are in a parallel circuit if I remember correctly. If you wish to let someone play their bass trough your rig providing for example you have the signal output connected from the back routed on the way to the amplification , a mono instrument can be plugged into the front TS 1/4 jacks on the front of the DS5R. If you switch to mono both sides will get the same signal down 3 dB I believe per parallel mult . You just might want to unplug the 5 pin from your Series I from the front of the DS5R first. I have done this before and it worked.