I would advise avoiding tube amps for the D5000. They need a full-powered solid state amp.
Tubes are fine if you choose a tube amp with ample power. There are several tube amps that will drive the Denons with ease like the Sound Quest SQ-84, Mapletree Ear+, Woo 6 and Singlepower Extreme.
I have owned all four and I still own the Sound Quest SQ-84 and Extreme. The SQ-84 drives low impedences with ease. But, the amp I absolutely love with low impedence headphones, actually any impedence headphone .... is the Singlepower Extreme.
From my ownership experience I rate them ....
1) Singlepower Extreme
3) Mapletree Ear+
4) Woo 6
The Extreme, no doubt, has the best sound quality of the four amps I mentioned. Furthermore, compared to the SQ-84 the Extreme has more tube options, is more space friendly than the SQ-84, I personally like the SP look the better and the Extreme is the most versatile amp sounding great with high or low impedence headphones .... should you decide to try some high impedence headphones at some point.
The tube cost of the Extreme is very reasonable especially if you invest in a tube socket adapter for the input tube so you can use several types of inexpensive, high quality nos tubes like the 6GU7, 6CG7, 6BZ7 etc... The SQ-84 uses 6V6GT's output tubes that are getting harder and harder to come by and which will easily run you a good $100 or more for a nos matched quad. The 6AS7 output tube used by the Extreme is plentiful and inexpensive at around $25-30 per pair.