bran_dongerImo the HD558's are significantly better overall. I have both pairs and prefer the comfort and sound of the 558s. These have more low end and a slightly smaller soundstage, but the sound isolation is much better. In the end it's your choice, but I don't think that these would be much of an upgrade.
bran_dongerI had both the 558's and AKG 240 mkii's and did extensive listening tests to see which one I liked better. The SQ was very, very similar - basically a wash. I ended up keeping the AKG's because they were slightly more comfortable. Less clamping force and I like the automatic headband adjustment. YMMV.