Each note has a "force" value which you can see in sappy. It's the second value on the track, right next to the instrument number. This is how loud the current sound output is in relation to the track's volume.
You can go into your midi and change each and every snare to have a lower volume, or you can just lower the track's volume overall (which keeps the contrast between snare and other drum instruments in-tact)
I'd recommend doing this though:
Find the snare instrument in the voicegroup, then lower its sustain volume. If the sustain volume is already very low, you can lower the attack volume instead.