How to edit/adjust the modulation wheel behaivor for any given voice:
1. Using the voice category buttons( [Piano] [Guitar & Bass] ) etc, select desired voice.
2. Press buttons either [A]-[J] whichver corresponds to the voice you want to edit
3. Press the  (up) button 'Voice Set'
4. Press one of the [Tab] buttons until at 'Controller' page.
5. In the 'Controller' page adjust the 'Amplitude' (this will remedy the volume drop), LFO (which will add some vibrato) etc as desired.
6. Press the [I] to save adjustments as a User voice.
7. Press the  (down) button 'Save'
8. Give a name
9. Press the  (up) button 'Ok'
Do you need an additional help?