Regarding the mixer. As you already have an x-pan, you could stick with the make noise schema for mixing and grab a moddemix to add to the case. I like to use the moddemix and optomix as a sub mixer before inserting it into the aux in on the x-pan. So rather than having 1 large mixing module, you can cascade submixers throughout the case and still have the benefit of the other functionality they add. And as mentioned above, maths is great for this as well.