hey guys looking for some help programming a mx 980 with a Denon 590

I have the software and while looking through the database of avr ir codes this 590 was not to be seen. So I asssumed the only way was to learn all ir codes into the mx 980 is learned. Anyhow while doing this noticed +vol ramps fast and -vol barely works after learned in. I was investigating this through a search and didnt come up with very much. Has anyone worked with this setup and help me understand what problem im up against here.