Hi, I have a new PC with good components. I recently added two monitors (amplified speakers) that give excellent results when I connect them to an amplifier, but when I connect them to the PC I listen to a pretty poor audio, not like the one I hear if I connect them to the amplifier. But maybe I connect it in the wrong way, in fact I plug the jack on the back on the light green connection that I believe is the connection of the headphones. But I don't see other options. Can anyone tell me which is the right connection for the components I have?

Here they are:
MOTHERBOARD: Asus PRIME X299-DELUXE
VIDEOCARD: NVIDIA GEFORCE GTX 1080Ti
Thank you