I got the AHD1001 last April, mostly to use in the bedroom, connected to Squeezebox via coax to Cobra D10. I've been really happy with them. Nice bass, light and so comfortable I can forget I'm wearing them, blocks out enough ambient noise that I don't realize if my wife is trying to talk to me. They did feel a bit delicate to travel with, but that's not what I got them for.
At work we're moving to a different floor and I saw what the work spaces will look like. They're more open, and I may no longer be able to use my Harman/Kardon desktop speakers as much. So I ordered the ATH-M50 for the exact same price as the Denon. First impression is that they are very heavy and sturdy, definitely more professional studio quality. They fold up unlike the Denon and are definitely more suited for travel. They have a protective spring by the plug, and the first half of the cord is coiled, which is good for avoiding getting it tangled in chair wheels and such.
Putting them on, they feel like a vice grip. Definitely not nearly as comfortable as the Denon. They do block out more ambient sound also, which is nice for blocking out the yammering around me, but bad when someone wants to talk to me as I get startled when I don't realize someone is there (I need a rearview trucker mirror!). First impression is the bass is even richer, but I don't have anything more specific until I A/B.
I'm concerned about the overly snug fit. Right now I'm using the box they came with to try to stretch them out. If it doesn't improve, I might switch them with the bedroom set, as I usually only use them in the bedroom for 30-40 min, while at work it could be several hours.
Amp - Both headphones are easy enough to drive and don't necessarily need an amp, but can improve sound a bit with one. For me a DAC/amp is essential at work because the headphone jack on the IBM has a horrible non-stop static. For $88 including shipping for both, it's definitely a great value. I would not want a higher end can at work because something like an AKG-K701 or Denon AH-D2000, 5000, 7000 is just too big and I would worry about leaving it in my flimsy locked cupboard.