After further exploration on the HK990, it appears that it has eq and bass management features that are in the digital domain only. As your are using analog input only you are not taking advantage of this, or if you are, you must be redigitalizing (spelling?) your analog signal to do so. I have noted that your amp is considered very good as an analog 2-channel amplifier.
So again, i believe you might find yourself very satisfied with your sonics with an improvement in your phonostage. You may later consider a cartridge upgrade also.