Each Part has its own volume, the answer is raise the Volume. There are no GM Presets in an S90 ES, per se, you are given a set of 128 normal S90 ES sounds + 1 GM Drum Kit that will respond when you send a GM RESET message and send the MSB/LSB 000/000 (normal), 127/000 (drum). In reality these sounds are made by the same tone engine as all the other Presets.
While it maybe true that not all sounds have an equal output level, but they all have a Volume parameter that you can adjust.
Adjust the volumes and [STORE] your configuration as a Multi.