I also own a 1650-AR. If I were to list its strengths and weaknesses as a stand alone player, I would say that the player is extremely detailed and has pretty good bass, but is deficient in imaging. So my experience with this player is completely different than yours. If your system is not detailed, I have to think its not the Dennon but something else in your system.
About a year ago I bought an Audio Mirror DAC and I continue to use the Denon as a transport. This is a much better arrangment than using the Dennon as a stand alone player. This improved the soundstage significantly and improved the already good bass substantially. I would say the combo exibits plenty of detail, but reduced a problem I had with listener fatigue. You may wish to consider this as an alternative to putting money into tweeking the Dennon. I think the Dennon still holds its own as a transport (it really is built like a tank). In terms of tweeking the Denon, I'm sort of with Myraj. For now, you might be ok with the Dennon as a transport and adding an external DAC. Otherwise, I'd sell the Dennon and put the extra money into a more recent stand alone player.