The Denon's headphone jack should output more voltage than the 55-Ohm Focal Clear's would need.
More likely the headphone jacks high output impedance is not a good match, for 55-Ohm headphones.
Maybe it also a current issue?
What sources are connected to the Denon?
Sorry for the silly question, so how would the HDMI audio extractor be setup. Where do the HDMI cables connect to and also digital audio.Quick look at the manual says these are for subwoofers only. I think you're only option is to get some type of HDMI audio extractor box with a passthrough function. Take your HDMI out and send it to the box. From there you can tie into your DX3 pro using digital audio.
Amir reviewed one of these boxes and found it passed through very good digital audio (https://www.audiosciencereview.com/...-blackbird-review-hdmi-audio-extractor.36478/)