If your looking for reasonably inexpensive, easy to setup and widely supported DACs I would suggest something from Hifiberry or Allo.
https://www.hifiberry.com/shop/#boards
https://www.allo.com/sparky/boss-dac.html
I2S boards like these attach on top of the Rasperry Pi without special tools or skills. Then connect to your audio system with standard RCA cable. Provides a compact setup but you may need to buy taller case.
Else you can buy a stand alone DAC which will take digital input via optical coax or USB. Will likely cost more but can be used in a non-Raspberry environment. The sky is the limit on prices but some popular DACs run ~100USD such as Shitt Modi. You might still need a board with digital output to connect to the stand alone DAC depending on its inputs.
If your Denon has digital input then you already have a DAC in the receiver. Then you would need a board that provides digital output to connect to the receiver or if so equipped try HDMI.