Yes, my D2000 sound better with an amp, and they sound better than the HD25-1 that I sold. However, my D2000 have an APureSound re-cable done to them, and they are closer to a D5000 in sound (based upon my trips to the local hi-fi store to listen to stock D2000 and D5000 vs mine).
With a D2000, they get better as your source and amp get better, and are truly Hi-Fi. The HD25-1 didn't have as much potential, even when I tried my APureSound HD600 cable on them they didn't scale up as well (the soundstage was really missing, and I was always more aware that I was listening to headphones).