I feel like Pro-MB could really benefit from an "Audition Triggering" button like Pro-C 2 and Pro-DS have. For those who don't know what I'm talking about, it's the one with the headphone icon attached to the threshold knob. Triggering it lets you listen to exactly what's causing the compressor/de-esser to trigger. This is pretty useful in a single-band context, but would be extremely useful when you're dealing with amplitude AND frequency-based thresholds.
This would also be a useful feature to have for Pro-G and Pro-L