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 in a live gig. I only used my HD500 to jam at home, or I took it to band rehearsal when I didn't feel like carrying my amp around. That's the only time I ever used it for amp modeling.
So I only have one question about the Helix. Is the Line 6 Helix a TRUE BYPASS unit like the M13?
I want to be able to have the choice to use the Helix in TRUE BYPASS mode when I'm playing live. With that said I would also use the full features of the Helix (amp modeling with effects) in the recording studio. :)
TRUE BYPASS? Yes? No?