For those that have any interest. I may be one of the few that have had the opportunity to listen to both the M1 and M3 side by side in a home environment. I purchased the M1 Turbo S in the early stages from Spatial Audio and Clayton was kind enough to upgrade me to the different versions of that fine speaker, each time there were audible improvements. When the M3 Turbo S came out I was able to order them and for the last month have been able to test them side by side with the older M1. Here are a few impressions.
Visual: The M1, as per specs, is shorter than the M3 and my black M1's had an offsetting grey grill cloth and of course the aluminum front panel. Personally I preferred the M1 visual appearance a bit more, but that is all personal choice and by no means is the M3 unappealing. The M3 series retains a slick modern look, in an all black finish and the legs of the newer M3 make the speaker much more stable than the single leg on the back of the M1 (hooray).
Listening Impressions: To be short, the M3 Turbo S in side by side comparison to the M1 Turbo, is a better sounding speaker in every respect. I believe that the additional work Clayton has done to improve the M series design has paid off immensely . The M3 with the drivers mounted higher presents a more realistic soundstage than the M1's. There seems to be better cohesion between the tweeter/mid driver and the woofer which results in a clearer midrange. To my ear the biggest change however was in the mid to low bass. With the M1, I was tempted to use a subwoofer (and tested it with good results). With the M3 Turbo I have no such desire.
I have not inquired what internal changes to the crossover were made to the M3 so I can't say if those changes were paramount to the improvements, but together with the driver placement, speaker height, integrated panel wave guide (all different than the M1), Spatial has hit the sweet spot!