Hi Beninho,
there are a couple of reasons coming to my mind that would cause problems like yours. Since you unfortunately didn’t exactly tell the details of your setup you need to decide yourself if any of the following might apply.
Turning off CEC may help but might not necessarily be your solution. You could first try adding “hdmi_ignore_cec_init=1” to the config.txt . This prevents the RPi from sending the initial active source message over CEC. This might be enough.
On the other hand some TVs (like mine 
) try to be smart and switch sources whenever they detect a new signal on any port. That can of course only occur if you connect the RPi to a TV and from there to your amp. I’m curious if anybody has a suggestion for this case, but I think, fiddeling with the Pi’s settings or even turning off CEC will not help at all – you’re simply out of luck.
If you are lucky adding just another line to the config.txt will do. “hdmi_force_hotplug=1” will make the RPi always send signals to HDMI, even if it does not detect any receiver on the other end.
I hope that helps.
Best regards