finally tried out how much an open back of my Code 100C would affect the sound of the amp. Surprise: The change is very subtle imo. The combo comes a little bit more "alive", a little bit more "airy", but the change was not so significant as I expected.
To avoid an irreversible modification I first just removed the whole backplate. Then - while I was playing - my son put the plate in and removed it again severeal times. "Open" the amp sounded as good as ever, (I didn't notice a change of sound) but when my son closed the back the sound became slightly "muddier".
Now I'm discussing with myself whether it's worth the effort to modify the backplate or not. If I do so I would only saw out either a rectangle 40 to 20 (or 30) cm or - alternatively - 5 or 6 holes with a diameter of about 10 cm.
I know there was a related thread here, but maybe some of you have made some further experience with this.
Maybe you should completely remove the original back and keep it in case you want to put it back on later.
It should be a simple and inexpensive matter to buy a small sheet of particle board or MDF, paint it black and make a new back with any holes in it you wish.