Velocity is NOT the way to adjust volumes in drum loops. Velocity is totally different from volume.
Different velocity triggers different sample, and it changes the whole nature of the loop.
A gentle hit on drum remains the same, gentle, if you add volume. If you raise velocity, it changes into a hard hit, which sounds totally different.
I'm a little surprised if the loops are so different volume-wise. You are only talking about Session Drummers own loops, not something you have gotten elsewhere? One rock loop should be usable with another rock loop without volume adjusting. A jazzy loop is then another thing.
I believe the solution, at least for the most, is adding a compressor.