Hi Tal,
when you're preparing drum samples for a sampler, and you have several velocity layers, the sampler is doing the velocity changes.
That means, all samples have to have the same loudness.
Let's say you want to make a snare sample with the 5 velocities that were recorded. You first normalize all of them, to 0 dBfs or to -6 dBfs, or whatever you want the loudest sample to be played with, when you play it with velocity 127.
If you now prepare one sample with full velocity you can play it from very soft up to the recorded loudness. If you build a sample with several velocity layers, it's exactly the same, only you get to hear an excerpt in the velocity you set.
This is how it works with the EXS24 and Alchemy, so I guess other samplers will work the same way.
Guido