So the K550's arrived today and I spent the last couple of hours messing about with them. Here are my initials thoughts. Please bear in mind that I have little-to-know experience with headphones as I've always been an IEM or speaker guy. I also do not have very many music files in lossless format. I've had my music for years and years, way back when 200GB of storage was an expensive addition to a desktop and FLAC was completely impractical for someone with 53 days worth of music. I also am only using the onboard audio of my motherboard, which is sufficient but certainly not up to where I want to be sonically. I'll be investing in a headphone amplifier and DAC combo when I settle on headphones.
Amp-less they are not amazing, but no frequencies are apparently lacking at first listen. I do find the mids slightly woolly at present, however that could be resolved with better audio sources besides mp3 320, a longer burn in time, and better amplification and conversion. The deep bass appears a little unfocused, but could again tighten in time. I have an issue with finding a consistent and effective seal, just like others here in this thread, and I imagine that would be part of the problem. I'll be performing the 'clamping mod' outlined in the OP and that should help with sealing and sizing issues. My ears get a little warm and stuffy, but I haven't felt like I needed to take the cans off. When I do, however, I feel the fresh air relaxing and a relief. The cable feels sturdy and is silent. The headphones themselves are built excellently for the €145 I paid for them. I would have preferred earpads that conformed to the natural shape of the ear rather than being perfectly circular, but they don't hurt my Spock ears so so far it's not a problem. Isolation is excellent, but I would never wear them out.
My biggest and only real complaint, is the weight and fit. I was hoping for a 'pillow on my ears' effect, but they're just too heavy and bulky to be described that way. The weak clamping force paired with the weight and size makes them a little... 'old' feeling. They don't feel like modern feats of engineering. Instead they remind me of 30-year-old headphones that my parents left me. When I apply a little pressure to the side of the cans, the bass, immersion and impact increases dramatically.
I have yet to test them with gaming and movies, two of my biggest uses for headphones, if not the biggest. I imagine they will be excellent because of the solid definition and the wide and deep soundstage—this is an assumption based on the way superior audio formats like FLAC sound clearer and deeper than HD mp3 formats.
Overall, for €145, they're good. I'll spend a few more days with them before coming to a more accurate conclusion, but so far for my first headphones I am both impressed and impassive. The problem is, there were so few headphones in my price range that met my criteria: Long cable, large earpads to fit my Elf ears, comfortable, priced no higher than £250, apt for gaming, music and movies, neutral sound signature with a blend of 'fun' and 'detailed'. Other headphones I'm interested in are the Mad Dogs, MDR-1Rs, KEF M500s, Philips Fidelio X1 and L2, NAD VISO HP50s, and a couple of others, both the closed and open varieties.