Hopefully I'm not too late to throw in my two cents and a couple of real-life stories.
I've been using the 840s for near 3 years now, and it's part of my office rig. I like its sound signature, with forward and very smooth mids, and not too much bass. This, of course, ties to my music preference, as I mostly listen to female vocals and pop songs. For classical music I use my home rig which is HD600 + Nuforce HDP (recently upgraded to HE-500). But I sometimes still like the vocals from the 840 better, although the HD600s beats them on any other aspect.
About a year ago I convinced one of my friends to join the audiophile world, and he came to head-fi for suggestions on his first serious headphone. He was greeted with an overwhelming recommendation of the ATH-M50, so he just rushed to amazon and grabbed a pair of those. Then he came to my office and we spent an hour ABing the M50 with my 840. Although he had almost no experience with high quality headphones, we easily came to agreement that M50 has better bass than the 840, both in quantity and in quality. However, my 840 destroys his M50 at midrange and highs with silk-smooth vocals and no sibilance at all. Soundstage was quite similar, though. Admitted, his M50 was not burnt-in yet, whereas my 840 had about 2 years on them already, but the difference was much bigger than burn-in effect alone. After the comparison he became quite disappointed with the M50, because his music preference was quite similar to mine.
Not long ago, another of my friend was also picking up his first pair of headphone. We went to H&R and auditioned M50 and V-Moda LP2. He's a basshead and really likes a lot of bass. And after some side-to-side comparison, he said the lp2 beats M50 on bass hands down. But my impression was that the M50 had some better instrument separation. However the bad thing about H&R is that we had to use their source and music, so I wasn't able to compare a lot of other aspects. He went ahead and purchased the lp2 later.
I don't have the 440s, so I wouldn't comment on them. Hope the above helps with your decision.