Maybe I don't understand, but if you have a bad tube all Mesa is going to do is help you order a new one, unless you mean it was bad when you got the amp from them.
I've had a couple of bad tubes from Mesa. I called them up and they sent the replacements and told me to just throw out the bad ones. Pretty cool since I did'nt have to mail back the bad ones for proof.
They will ask you to switch your tubes in and out because they are'nt going to send a new quad or set if its one bad tube. So you have to work for it a little.