The SM9.2 is one loud SOB and so is my M9....SM.9.2 seems to have more headroom at 8 ohms than my M9.. At least mine does.
The SM9.2 has near surgical precision in the EQ section.
The M9 has many EQ options with the GEQ and roatary presets. But these amps are two different animals.
The SM9.2 has an out of the box warmth and needs to be tweaked to get a raw rock tone..
Where the Mesa has a raw rock tone out of the box and needs to be tweaked to sound warmer. IMO.|
The low end on the M9 is massive when set flat.. and the SM9.2 needs to have the Global Bass engaged to get that low..I personally like the low end girth the M9 has in native settings.. It reminds me of when i used a full rack system with an old API or Neve Preamp and crown PSA2 poweramp on my bass rig.. The M9 is sort of a rack system in one self contained amp.. the SM9.2 is like a channel strip with its two channels
Both have their sonic signatures and both are equally good in the right environment.
Both amps took me some time to figure out what i like and how to get to that place once again when going form gig to gig and large room to small bar gigs.
The M9 took a little longer with the Rotary EQ. Because i like all of the Rotary settings! All 5 sound great.. And i would mess with them from song to song.. Just to be a tweakeraholic..

The compressor in the M9 is one of the best comps in any head available. Love it.. and i only really use it at times. but it is smooth and quiet!
I like the Combo Nuetrik speaker outs on the Mesa.. It saved me on a backline gig when all they had were older hartke cabs with 1/4" jacks..
These amps are at opposite ends of the tonal spectrum... Both equally great..
But like i said at 8 ohms my SM9.2 has more headroom than my m9... At 4 ohms, i never got to crank either up that loud without the risk of being fired.
M9 is a beast, and will rock the room.. You feel the punch in your chest... It DEF moves some air