Yes, I would imagine that to be true. There is nothing wrong with the K550, it is a fine headphone. I enjoyed my time with it and liked the sound, except maybe for the bass output, which I felt was a little low, but the bass that is there was some of the most accurate and musical I had ever heard. It simply came down to the fact that for me the 550 was just too bulky and did not provide enough clamping force, for my head, for portable use, which is my primary need for any headphone.
Yes I like the HP50 more in almost every respect, but I would certainly not dare to suggest that it is worth twice the price of the AKG's, as it would be in your case.
I was just trying to give you some suggestions, as you asked, and didn't realize that you were based in the U.K.