Hi all,
FWIW I've been struggling w/ the sidechain/2600V3 in Logic for a while .. and I actually DID get it to work a while ago (last year sometime?). I just recently had occasion to try to set it up in MainStage to no avail, but I knew I had it working in Logic, so I went back to that project, and alas, it had stopped working. :\
I've spent the past few days trying different things .. and no luck with the 2600V, but it DOES work with the MiniV3 .. in fact, multiple ways - I can just pick the instrument channel of the source in the side chain drop down, or send that channel to an AUX and pick the AUX, .. it just works.
Also, I can get the 2600V side chain to work in Ableton Live 11, but interestingly it gives you 2 options of where to send your source audio from the sources' channel. In Ableton you send the source audio track to a destination in the output of the channel, in which it gives you 2 options for the 2600V as a sub-menu when you pick that inst. channel .. a stereo audio in, or a side chain audio in, and it works only when selecting the side chain audio in. (obviously you have to set the 2600 instrument to enable the side chain) It DOESN'T work this way for the miniV in Ableton - you just have to pick the miniV channel in audio out of the source .. no sub-menu, so I can only assume that it's implemented differently in the 2600V, and in a way that used to work in Logic, but an update of Logic within the last year broke it.