I've been using the M13 for 5...6 years now. The thing I like most about the M13 is that I can run my effects in front of the amp like a traditional pedal board.  I like the fact that my M13 is a TRUE BYPASS unit.     I also have the HD500.  My only complaint about the HD500 was that it wasn't a TRUE BYPASS.  I'm not interested in the amp modeling like some guitarists are.  I just want to use the effects only without the HD500 coloring my Mesa Boogie Dual Rectifier's tone.  So I never used it