Not sure if anyone answered this... The diff is the Mach1 comes with foam ear plugs and the Mach1c includes custom silicone ear pieces from Sensaphonics.

I highly recommend the custom ear pieces. Sound isolation is quite good and they are very comfy. Only issue with them is they have to be thoroughly cleaned often or they'll fall off the headset.

Speaking of... does anyone else here have the custom ear pieces? How do you guys clean them?