I just got my used Mark iv and have to say it`s the best amp I have played(only been playing 1.5 years).My question is I also have a Line 6 M13 which won`t work with this amp,I have set it up the same a my Peavey Valveking and it work`s fine in that amp.I`am useing the footswitch and that turn`s the loop on but no sound at all and leaving the loop on at all times I still have no sound at all.I hope someone is useing this pedal so I can get some help.