aux in is for outside sources like music to play along with - it is only controlled by master and bypasses the preamp controls.
I guess you could use it the way you describe but if I needed that functionality- I’d recommend going to the 800 which has a proper send and return as well as aux and a very good drive channel.
I’m planning on going to a town about an hour away to try both of them out when I get a chance, but I doubt I need the 800 for my purposes. There’s quite a difference in price between them over here so the aux in as a return is not really something I need but more like a “handy if it works” kind of thing.
