I also have a Mut@nt HD51 and it works OK here with a Sony TV and Yamaha AVR. As already mentioned HDMI-CEC can be a hit & miss affair with some devices cooperating nicely and others refusing to do so. An approach worth trying is to power down the TV - pull the plug. Then power up the Mut@nt and disable, then re-enable HDMI-CEC. Finally power up the TV and see if the HDMI-CEC handshake between the devices has been initialised successfully.
There's also a plugin to allow testing of HDMI-CEC signals, but I haven't used it.
Good luck.