I had to bump up this old thread to share my experience with the K550.
I bought my pair used. The first owner sold them as too bass light and canny sounding with spikes in the treble side. I too thought they sounded terrible when I first got these. Comparing them to my Grado SR80's made me go back to the Grados every time. But with a significant amount of time this has so totally changed!
Now the SR80's sound absolutely distorted, sure with nice bass, but not much of a sub-bass at all. The K550's seem to sound exactly as I could dream a pair of headphones to sound. And they really don't feel closed back phones at all.
And I do think the pads need to give in at first. At first there's a noticeable difference in the sound, when you press the cups against your ears. They sort of needed the pressing to make the seal. Now pressing them doesn't actually change the sound at all, so there is something there that's changed for sure.
Just came to type this in, wishing someone disappointed in K550's would read this. In my experience they are a pair of headphones that need a significant period of burn-in!! They really are a beast. Peace out!