The only way I have found to get the BL3500 into A.Aux mode is to switch the BS4 to A.Aux mode, then switch the BL3500 on using the physical Mute button next to the display.ย I can then mute the BS4 if I only want the sound from the BL3500.ย It works, but itโs not very practical!
You could try adding your BL1611 to the same ML network as the BL3500 and BS4.ย Connect your WIIM Pro streamer to the BL1611โs AAL input (audio must be to pins 3 & 5), and then try to select it using a Video source on either the BL3500 or BS4 (again BS4 should be in A.OPT 2).
EDIT:ย What I am proposing is a bit like this OneRemote setup:
