Thanks to all for your responses, sorry I've taken so long to come back. Seems to me I didn't phrase my question too well, but before I explain a bit better, the good news is I have managed to get Monster Drums working correctly(?). But I seemed to have to do it by going through the process in Audio Tech TV's "How To Setup Multi Output Instruments in Cakewalk" Tutorial i.e. insert an instrument rack for MD, add several tracks for each type of drum instrument (with output to the created stereo "Drum Bus") etc. like below:
Then add Midi tracks per drum instrument like so, with output to Monster Drums:
So this works fine but seems like a lot of work to add all these 7 extra tracks to an existing Midi file, maybe I can create a template project and suck in the old Midi tracks to the template. Anyway, I'll play with that.
As suggested by mettelus, I tried just inserting MD as a "simple instrument" in an existing song, but only get a kick (I think), no other drum sounds, I tried fiddling around but eventually gave up, all too hard 🫤
I will start using drum maps, though: good suggestion.
For background, the old Midi songs I am editing were created in Cakewalk 9 using Roland (Endoril?) Virtual Sound Canvas as the "synth", with Channel 10 being used for GM drums. The songs were only used as backing tracks when I used to do one man band gigs, but I use them now just for practicing new songs at home. So that's it, nothing too complicated.
Sorry, this got a bit long, thanks again for the help, guys.