I went from the D2000s (very similar to your D5000s) to the HD650s. I haven't looked back.
I loved the Denon sound signature for a long time. It's so lush and full sounding. The bass is powerful. The highs are sparkly. But after I listened to the 650s for a while, I just see things differently. The mids are non-existent on the Denons. The bass, while nice and powerful, is actually sloppy and boomy. The highs sound brittle.
The HD650s are just great. I have the newer version of them and they don't have that dreaded Sennheiser "veil." The bass is detailed and very accurate, in my opinion (I'm a bass player). You don't get that slamming bass like you do with the Denons, but the Senns have the right warmth. It's like you can actually hear the timbre of the note—the bow on the resin, the fingers plucking, the mallet hitting the skin. It's a different kind of warmth.
The mids are so much better on the Senns. More present, more accurate. I could go on and on but I won't.
Some people say the highs are rolled off with the 650s. I don't agree. I think most people like artificially boosted highs, which I find shrill and brittle. The 650s have very nice and smooth highs.
In the end, breaking down the frequency spectrum just isn't accurate enough. You need to look at the sound signature as a whole. Are they both warm? Yes, but a different type of warmth with different levels of detail. Don't get me wrong—I love me some Denons. They have their own sound that I really do love. But I've come to love the Sennheisers much more.
