Concerning the 4-pole filter: you are right, of course. Had a closer look at the MIOS project linked above and saw that he's basically replacing the output VCA and parts of the filter circuitry to be able to modify the filter characteristics. 

 

Concerning the M6: There are obviously a couple of differences in the schematics , e.g. the 6522 VIA which is absent from the M1000, but I guess they kept the basic design. And the M6 schematics scans are slightly better to read.