I've got a Mini Rec and can't say that I've had that issue.
Time to troubleshoot. Change only one variable at a time, and always start with the simplest "fix" first.
First, see if it makes any difference whether or not anything is plugged into the amp. If it stops humming with no guitar and/or lead plugged in try a different lead. If swapping leads has no effect, try a different guitar if you have one.
If it hums regardless of any of the above, try another electrical outlet/different room.
If none of that changes anything, try it some place other than your house if you can.
If none of that has any effect, call Mesa.