But it seems that this doesn't turn off CEC completely as volume control via CEC still worked. Although it's off by default on mine because it's unstable, I enabled it to see if it would still work. So "Control TVs and Recievers" seems to only disable most CEC features, but not CEC in general.That is the ATV's way of turn off CEC.
But shouldn't every TV with CEC support be able to hand over volume control to an AVR? I have a Philips TV and a Denon AVR. When the AVR is on, the TV is set up to mute the internal speakers and hand over volume control to the AVR. So if it works, I can control AVR volume with the buttons of my TV, AVR and external devices like the ATV. But if it's not, it's just broken in general. Then the volume buttons on ATV do nothing and the ones one TV/AVR just adjust the device's volume. Then I have to turn CEC off on TV/AVR, power both devices off/on and enable CEC again to get it back working. So I stopped trying, disabled the TV speakers completely and now only control volume on my AVR over IR.Volume control via CEC is only guaranteed to work one way down: from TV remote to the rest. Some brands implement bi-directional CEC volume control. Some don't. In my home, Vizio TVs won't let you do CEC volume control from streaming boxes. Samsung and Sharp TVs do support such.