hd700 is pretty much a step up. it goes much deeper into recordings and bring out lot more details, although mostly are flaws in recording. a lot more transparent in that sense. also offers more clarity, while sounding natural like hd518. akg has a bit accentuated treble in comparison.
that said, its quiet a bit higher in terms of resolution. really cant listen to more than 1-2 hours at a time. i guess i will have to get used to it.
for that, k550 is a lot easier to enjoy. I can listen to it 8 hrs a day. also akg has that treble coloration while hd700 is more neutral, that gives guitars, vocals and string instruments that edge, that I like a lot. in contrast hd 700 is lot more natural and convincing presentation.
k550 is a little closer to akg k701 level I think. and that pretty much is sweet spot in terms of price to performance ratio as well as most enjoyable sound. i got 700 for under 30 k from flipkart. for that price its very good deal. its twice performance for twice price over k550 easily.