on the link below, check out the "amp" folder on the left and the photo should come up. can you tell which model it is? I'm not very knowledgable about all of the mesa gear, but I don't recall one that has a simul-class switch on the front. thanks for the help.
http://www.shutterbook.com/albums/busdriverstu
http://www.shutterbook.com/albums/busdriverstu