If all else fails go to a Music Store, that stocks the MXR, with your accordion ,and ask to try it in- store....good retailers will let you do that particularly if it's a slow part of the day or week....they may even have a room set up.
Appreciate the comments - the GE7 is definitely more widely used I'm sure.
My boxes are C and D only, so my interest is tweaking for best sound rather than accommodating accordions in a bunch of keys. I'm thinking real seriously of giving this MXR EQ a try.