Have you ever oiled the bearing of your Denon? According to the manual for my SL 1210 (another DD TT), the bearing should be oiled about every 2000 hours (which would be about 2 years for me).
As Piedpiper asked, What do you hear (or not hear) in your current rig that you'd like to improve? That would determine what to do next. Some kinds of things can be improved with maintenance, vibration control and isolation, a better mat, or a better cartridge. Only then would you reach the limits of the drive system or tonearm.
Given that the better DD drive systems are consistent and quiet (and the Denons are definitely counted among the better ones), and a cartridge acts like a microphone when it's in contact with a record, the noise/vibration/resonance control would be the most important thing to take care of first. If you don't and then you upgrade, those problems migrate with you, and they have a profound effect on dynamic range and clarity. In fact, you would want to make sure that's taken care of before upgrading the cartridge, because it'll emphasize the resonances if they're not controlled.
