Thanks for that Gray, that looks like a workable solution.Well, as you're into low-cost tinkering....
A dual-gang logarithmic pot would give you control into the Fosi input -and parallel to sub input.
Here's one I made earlier, to use as a single-knob control for the Lepy amps on the back of my Kefs:
View attachment 5215
There is one proviso, the Dali's+ Fosi + Denon was set up to be Wife Friendly HiFi.
You know, 2 buttons to switch on power and a big dial for volume control, eject button for the CD; that neat little gadget would send her into a fit. 🤣