HM5s really aren't as flat as people make them out to be. Check out the
measurements for the FA-003 (same can) on Innerfidelity. They are NOT hard to drive.
If you can find a way to afford the $200 open box D2000 on Electronics Expo, go with that. Otherwise, I've only heard the HM5 and M50 out of all of those.
I think I preferred the M50 over the HM5, but it's been a long while since I've heard the M50. Most people seem to enjoy the HM5 more than I do as well. I don't know...I just thought the HM5s sounded weird. They're kind of laid back and dark, kind of bloated sounding, and have a weird mixture of sounding smooth and not-smooth at the same time. They have a weird dip right around 300Hz, which I found to be very noticeable along with the emphasized lower mids and bass.
Again, though, I seem to be the only one that thinks this about them. Granted, I was comparing them against my D2000 and modded T50RPs. When I took a break from headphones and came back to the HM5s, I found them pleasing to the ears until I put my other headphones on. I think they're semi-competitive in their price range, but I found them to be a step down from the HD598.