This still doesn't answer one of my key questions. How do they compare to the HD555? I want to know if doing this little upgrade will pay off at all.
I got around 200$ and I was a bit shocked by seeing most the discussions revolve around way more expensive headphones. I think it's only fair to investigate a bit before spending them.
As for what music I intend to listen to, it's more often than not of the noisy, harsh and distorted type. Definitely guitar-driven stuff most of the time.
There are plenty of discussions centered on low and mid-fi headphones.
As for the M50 vs the 555, the M50 will be superior in basically every way. Still, the best advice I can give you at $200 is to try and find a pair of D2000s on the For Sale forum. I prefer them to several $1000+ cans.