As I mentioned, I don't recall having to turn CEC on in order for ARC to work on this LG with my Denon x4200w, but I may be fuzzy on that. As for sonic differences, I'd say that in my room the 3070 does have a bit more spaciousness and oomph than the 2080 with an 8-point YPAO calibration done on both. I'd like to keep the 3070 just for its BT connectivity, which Yamaha decided to remove for the 2018 AVENTAGE receivers, but I hope to get these two issues ironed out before I make my final decision.ARC is part of HDMI-CEC, so it is normal that HDMI-CEC must be on both TV and AVR for ARC to work. However, it doesn't need to on for source devices like BD player, gaming console, or cable box.
BTW, do the 2080 and 3070 sound different once setup identically?