Is this the current incarnation? I noticed three examples under this name. I'm a noobie so can't give technical answers, but you mentioned a mixer with switches. The Synthrotek MST 4-Channel Audio / CV Mixer has an On/Off switch for each channel. it's only 8hp so still leaves 11hp for a Quad VCA or other toys. Good luck with the build. Keep Patching.

There is a music shaped hole in my soul that needs patching up...