Keep in mind the M6 does not come with a foot switch but the M9 does which is great for muting as well as switching in and out the eq if desired. The M6 has a ton of switch jacks on the back so you can switch between the 5 tone presets but you need some kind of multi switch or a bunch of individual foot switches.
The M9 has a toggle on the back that allows you to defeat any of the presets when activating the graphic eq, or leave it so that the graphic is added to the preset sound when kicked in. Very cool feature! I currently have mine set to preset number 2 which has a bit of a mid scoop for a modern deep tone and then can foot switch to number 3 which is flat and the graphic tweaked for a bit of a mid hump which works wonders with fretless tones.
I thought the M6 looked meaner with the big knobs and simple layout but the M9 is the way to go for several reasons.
The M9 has a toggle on the back that allows you to defeat any of the presets when activating the graphic eq, or leave it so that the graphic is added to the preset sound when kicked in. Very cool feature! I currently have mine set to preset number 2 which has a bit of a mid scoop for a modern deep tone and then can foot switch to number 3 which is flat and the graphic tweaked for a bit of a mid hump which works wonders with fretless tones.
I thought the M6 looked meaner with the big knobs and simple layout but the M9 is the way to go for several reasons.

