Even an inexpensive tube preamp will probably increase the level of refinement of your system over the Denon.
However, if your system can accommodate it [i.e. enough output voltage from the source and input impedance of the amp] an inexpensive passive preamp might fit the bill as well.
For the last few months I have been experimenting with the Antique Sound Labs T1 DT and a Luminous Audio AXIOM passive preamps along with my reference Blue Circle BC21 tube preamp. At $399 and $150 brand new respectively, they are both quite the bargain, and have shown me what the Blue Circle has added sonically to the overall presentation.
Both passive pres mate very well with my other components and pass along a very full, dynamic and extended sound. However, the warmth and dimensionality of the BC21 is missing from both.
But at $150 [I paid a few bucks more for the upgraded resistor] I'm definitely keeping the AXIOM. I like it a lot!