I had searched up and down on the internet for anyone who was able to get the Denon AVR-S510BT to fully function with an X1 remote. Most people either enter a code where the volume can only change with one button push, or a code where the Denon will only turn off but will not turn on.
For this receiver, Denon code 32516 seems to work on all functions. Power on, power off, and full normal volume functionality. Enjoy!