I'm guessing you're actually more concerned with VELOCITY than VOLUME.
I've had the same problem with drum grooves from various vendors, and here's my fix. Not the only, maybe not the best, but mine...
1. Highlight the MIDI clip that you want to amend.
2. In PRV, make sure your cursor is set to "Move," find the Velocity bars in the bottom PRV Window (there are 3 windows in my PRV - Drum Map, Piano Keys, and Velocity graph).
3. Raise (or lower if needed) that velocity bar until it's in the same range as the "ideal" clip. If you've selected the whole MIDI clip, all notes in that clip should change their velocities.
Since velocity-sensitive synths will play different sounding notes with higher or lower velocities, this will give you a consistent sound and level out the volume.