I know that the M13 is pretty sensitive to cable directional layout. If they cross AC power cords they can pick up noise. Also, sometimes noise from other power sources come through the front end of the M13. It could be your EX7 was introducing some of that. Make sure the output of the EX7 isn't too hot for the M13. I've yet to run my EX7 into my M13 but that is my ultimate gig solution.
I'll post your info at the M13 forum and see what responses we get.
EDIT: The HRDLX is prone to input jack problems. I've had to fix a few and they sound like what you describe. It's possible that unplugging and replugging into that jack temporarily negated the issue. I've also had that experience with tubes that were about to go out.
"No Tele For you." - The Tele Nazi
Ha! Tele-ish now inbound.