The M50 is a pretty solid headphone, and you might not find much of an upgrade in the closed category for under $300. It measures well from a frequency response standpoint (fairly neutral with some rough spots) and does not have much in the way of resonance problems. I have heard they revised the M50, which if true, this might not be the same for your particular M50.
I have not heard the K550, but it does seem to have a large treble spike right around 7-8KHz along with sharp resonance problems in the treble according to purrin's measurements. Tyll's measurements are slightly different, but I personally have found purrin's measurements to be more accurate when it comes to mids and treble. Aside from the treble issues and a dip in the 3-5KHz range, they seem pretty nice. However, I wouldn't recommend them at that price based on that data and others' impressions of them. I would encourage you to sample these with your own sources and material before buying.
Another potential option is the Brainwavz HM5. If you aren't aware, it's the same headphone as the popular FA-003. I believe they're around $140. I was personally not a huge fan of them, but the vast majority of people seem to really like them. They're supposedly fairly neutral. I think it would come down to personal preference whether or not this one actually beats the M50.
If you can find it in that price range, there's the Denon AH-D2000. They're known for their strong, extended bass. I'd say they're somewhat bloated in the bass, and they have some spots in the mids that are a bit recessed. Treble can be a bit sibilant, peaky, and has some sharp resonant issues. I personally liked these better than the M50 when I had them in their stock form, but my opinion might have changed now that I'm getting better acclimated with what a neutral, reference sound is like. I think it would be fair to say that the M50 is better than the D2000 in many, if not most, regards.
As for the Mad Dogs, those look like a great deal at $250. I've modded the T50RP before, and I have a set of the Paradox. I'm somewhat familiar with them and have heard their potential. From a technical standpoint, they should be quite good. They are voiced more like the HD650 or LCD-2, in case you do or don't like that sound signature. So, sound signature aside, they're probably better than the M50 in most ways. That's just my guess based on my experience with the T50RP and measurements I've seen for the Mad Dog. Whether or not the E11 would be good enough for them, I don't know.
Still, from a technical standpoint, the M50 seems to have less issues than most of these headphones in regards to frequency response and resonance. What you ultimately prefer, however, is up to you.
