Which should I buy ?
Beyerdynamic DTX 501 P
I don't own either of those products but if its any help I own headphones made by said companies, for your needs I would probably choose the AKG's. Their god like clarity will be an excellent way to hear your music; but the beyers might be a little closer to the overall warmer sound of your Denon's. All the AKG's I've heard thus far seemed a little bass light.
Wow I was at the same crossroads less than a week ago, although I had also considered the Audio Technica ESW9. Have you read my review of the DTX 501 p? http://www.head-fi.org/products/beyerdynamic-dtx-501-p/reviews/7671
Anyway, I have not tried the k451's but I have heard chatter that they are the same as the Quincy Q460 and I did try those on at the store. If you look at the technical specs side by side, they are near identical if that makes any difference. They were using some dvd of a live music concert and I don't think the audio source was good so I couldn't tell you anything about the sound. But the fit was tight for my large head. and they were a bit smaller and more fragile feeling than the dtx 501 p.
The DTX 501 P's are fantastic so far. They are great with jazz and pop music cause the bass if surprsingly punchy but doesn't overwhelm and the mid's/Highs are balance and very clear.
Perhaps you can buy both then try them on and return the one you don't like?