I know those bands well and have a pair of K550's.
One of my initial gripes with the sonic signature of the K550's was the slightly woolly midrange. It didn't feel as focused and natural as I would have liked, especially for the midrange-focused music I commonly listen to, which are all centred around guitar and vocal performances, both midrange instruments. I don't know whether it's because of that, but the sound of the cans can be a little sterile and unapologetic. I don't necessarily mean clear or precise, which they are, I mean to say that they are too harsh and clinical at times. However, the incoherency in the midrange did settle after a few weeks of burn in. My UE700 IEMs are clearer, but also warmer and more supple, which I prefer.
For Opeth, Mastodon, Tool, this is fine and not a major concern. The K550's have a tight bass with a subtle but incredibly beautiful sub bass that I love hearing in bands like Electric Wizard and YOB. It gives the sound a lushness that is often lost with the muddy midrange and clinical highs. A proper seal makes this even more evident and important. Also, a good amplifier and DAC will definitely improve their performance. They both add thickness and fullness that can be lost through a poorly sealed 'phone. Since I currently do not have access to a dedicated amplifier/DAC, I use an EQ to add a little low-mid bass and increase the volume. It helps fill in the gaps, especially for Doom.
Overall I'm happy with them, but I will be trying other headphones in the future. For a variety of applications, they are perfectly adequate and sound very good. However, if you want deep, lush bass then consider the V-Moda M100's. To be honest, though, I think you'll be better off with a 'better' (read: clearer) headphone like the K550 and a nice headphone amplifier and DAC. Make sure you're rocking lossless audio and you'll be set. Another option I didn't consider before buying the K550's are the SoundMagic HP100's. They supposedly have all the benefits of the AKGs, but with added bass texture and depth. I kinda wish I had gone for them, but like I said, I'm happy. Really, for me, open headphones are the way to go, but sadly I need the isolation.