There's so many better headphones out there than the M50. The problem is just that nobody seems to want to take any effort to find them and would just rather go with whatever else someone says is "Best". I perfectly understand this and not everyone does this. Why do you think so many great headphones are ignored forever? If you check out ljokerl's reviews, you can find many just as good as the M50. It's best to just find the signature YOU like and find a headphone to match it. I now rarely go by reviews anymore. If I did this I'd have skipped out on many of them.
I personally don't understand why people call the M50 balanced. Bloated bass, slightly recessed mids and rolled off highs (my pair, seriously) is balanced? I did like my pair a lot, but I was never impressed with them at all because it's just not my type of signature. I do understand the white box version is much brighter. Based on sound, I'd take the SRH-840 over the M50 any day. It's too bad the comfort is terrible. Now as for the 440 or the M50, maybe the M50. Only because the 440 was kind of too bright for me. It did have a better fit than the 440 though.
The headphone that's better to me in nearly every single way compared to the M50 is the Koss Pro DJ 100. Better bass and better mids, soundstage and imaging. They're also built better and are much cheaper and smaller. They even kind of fold up. For being only $80 they have a ton of detail. It's the first cheap headphone that made me believe an $80 pair CAN compete with $150-$200 headphones and price doesn't always mean everything. They're even better than my old favorite the SRH-840!
I actually think the D2000 is more neutral than the M50. This is only when amp. It's bass when amped with a desktop amp doesn't feel excessive or bloated in any way. I actually would never use a D2000 unamped though..ever.
Another headphone I'd take over the M50 is the AKG K240 Studio for $99 or the new Sennheiser HD-598. I paid $170 for mine and it's sure to come down in price. Nearly everything is good on these to my ears. It's soundstage and imaging is the best part. Too bad they're so ugly. The DJ100 is better in most areas except for comfort and the soundstage. Level of detail is much higher on the DJ100, but the HD-598 isn't too far behind. I love how it doesn't seem to be as "laid-back" as the other Sennheiser headphones i've owned. I hate to say it, but I might even like my HD-598 more than my old HD-600!