This is a pretty typical problem. There is no universal standard for scaling CC values to devices, so a given CC91 value sent to one synth may result in a different reverb level than when sent to another synth. Consequently, you normally have to edit CC91's when adapting someone else's MIDI file, or when changing instruments.
What you want to do is open the Event List and click off the Note button at the top (far left), so that notes are excluded from the event list. This will leave only CC events showing. Look for CC91 events and delete them. Once they've been removed from the track, you'll be able to automate reverb with an envelope, or set its level via the track header "Rev" slider, or just use the MO6's controls.