Some presets in ZENOLOGY GX are set up with a wide velocity range and lower output volumes to accommodate large polyphony. In 1.9.2 we boosted the overall output volume as well as setting a minimum velocity setting for the on-screen keyboard in ZGX that should help the overall output levels on all presets.
Note: Be sure to update to the latest version of ZENOLOGY GX from the App Store.
To access the Minimum Velocity Setting, go to the Main Menu and go to Settings, and tap on the MID tab.