bitflipper
For adjusting velocity and duration, click on the associated "key" in the - what do they call that thing - the vertical strip on the far left of the PRV that looks like a keyboard if you don't have a drum map, or contains drum names if you do. That'll select every MIDI event corresponding to that note, and then you can adjust them all at once.
And that is just about the only use I have ever found for that "feature". But if you're not looking and paying close attention, it will get you in trouble too.