In the voice edit menu - there is a velocity parameter for each voice in a preset. If this parameter is set to "ON" the velocity is touch sensitive - since your Midi accordion is likely sending fix full values - the volume will be as you indicate "very powerful" - You can change this value from "ON" to any velocity value from from 1-127 - try picking a velocity of 75 and see if that solves your issues. You will have to do it for each voice in a preset you want to use.....

I also recommend you join this group as well.... lot's of great information and questions are answered almost immediately.

https://www.facebook.com/groups/KetronSD90/?ref=group_header