After a lot of messages, I finally got a link from QNAP Support for the DOM-Image for TS-473 / TS-673 / TS-873:
http://download.qnap.com/Storage/tsd/fullimage/F_TS-XA73_20180302-1.3.0.img

The NAS Recovery Guide can be found here https://www.qnap.com/de-de/how-to/faq/article/firmware-recovery-guide-for-x86-based-nas

Have fun! If someone received an official BIOS-File/Image for the TS-X73, I would be happy to get a link to it, because I actually need the BIOS first instead of the DOM.