Its a combined system. (NAD M2 for 2 channel via AES/EBU & Parasound P7 surround preamp outputing the .1 to the same sub and the left and right surround to NAD M2 via interconnect) So, technically I don't really need simultaneous use of line and speaker level inputs. However, if the manufacturer won't certify this particular use, I could destroy my amp with conflicting grounding. That is, i would have a connection coming from the NAD and different connection coming from a surround preamp. Also, I don't know how the sub would handle two inputs coming in at the same time. So, you are correct that I don't need to listen to line and speaker level at the same time, but its going to happen anyway when I am using the NAD M2 for my surround fronts and my preamp is sending LFE at the same time.