If I take the back plate off of my JCM900 head, nothing will happen will it? Now it is directly showing the tubes in the open space, is this ok?
no, it will be fine. Everything is connected securely, the back plate only protects the tubes from getting smashed accidently. It's probably better for the amp with the cover off since it will run cooler.
My mesa is completely open back except for an aluminum bar across the back.
Taking the back off a speaker cabinet can change the sound distribution a bit, but taking the back off the head should be fine - even beneficial, as stated above.
Alright, that's what I was thinking in my mind, just wanted to make sure that something weird wouldn't happen.