To make the DSM-1 equivalent to the DSS-1, you would need to do the following:
-Modify the analog board with extra sample and hold IC’s and analog switches, so that control of the filter slope, mixer, and resonance would be possible.
-Add two DDL circuits (with associated control lines)
-Implement a routing / switching circuit so that two voices feed one NJM2069 filter chip
-Rewrite the software
Does that mean that a DSM-1 upgrade is unlikely?
Not unlikely but will have to wait until I get some free time.