I was able to get one set of "IR Codes" for the Denon AVR series from Global Cache Control Tower; however, their Hex Codes appear to be different than what Tasmota uses for "raw data", so not sure how to utilize their data with Tasmota.
Not giving up, I did program a spare Dish Remote's "Aux" button with the Denon code such that I could use the Dish remote for limited controls of the Denon AVR. I then used one of my spare Wemos D1 with IR Receiver and read the IR codes being sent.
This is just a limited set of IR Codes and the initial value of "Repeat" was 0. I have to set "Repeat" to 1 in order for my Tasmota IR Send commands to work.
Denon AVR-391 IR Codes (as retrieved from a Dish TV remote)
Power {"Protocol":"DENON","Bits":15,"Data":0x220C,"DataLSB":0x4430,"Repeat":1}
VolumeUp {"Protocol":"DENON","Bits":15,"Data":0x223C,"DataLSB":0x443C,"Repeat":1}
VolumeDown {"Protocol":"DENON","Bits":15,"Data":0x213C,"DataLSB":0x843C,"Repeat":1}
Mute {"Protocol":"DENON","Bits":15,"Data":0x203C,"DataLSB":0x043C,"Repeat":1}
ChannelUp {"Protocol":"DENON","Bits":15,"Data":0x1A6C,"DataLSB":0x5836,"Repeat":1}
ChannelDown {"Protocol":"DENON","Bits":15,"Data":0x196C,"DataLSB":0x9836,"Repeat":1}
SAT/CBL {"Protocol":"DENON","Bits":15,"Data":0x204C,"DataLSB":0x0432,"Repeat":1}
DVD {"Protocol":"DENON","Bits":15,"Data":0x231C,"DataLSB":0xC438,"Repeat":1}
Tuner {"Protocol":"DENON","Bits":15,"Data":0x228C,"DataLSB":0x4431,"Repeat":1}
I will most likely spend the $15 and get a replacement IR Remote specifically for the Denon in order to be able to obtain all the codes.