A quick fix in sforzando ;
Drag the sfz onto the GUI and then select the CONTROLS tab and turn the volume knobs up.
But that is still too damned quiet for me, so close whatever you have with Gentle Vibes sfz and right click on right-click on the sfz and select: Open with… (use Notepad)
In Notepad search for the three volume settings on 0 and change them to say 25. (the higher, the louder it will be later)
Save the changes as an sfz.
Load the sfz into sforzando and go back to CONTROLS tab and turn the volume knobs to get the mix you are after.