really? i had no idea.. im a pretty big noob when it comes to these things, this is my first real tube amp and i have only had it for about 4 months..
I wouldn't be plugging in any tube amp without a ground. Your power cord should definately come out, I've never seen any Mesa that didn't and I've owned 4 Mesa heads.
my only problem is that i live in a small town where the nearest tech would be 2 hours away and i wouldnt get my amp back for like 2 months