I am pretty familier with HD800, also had them for weeks next to my K702, both have their pros and cons, but at the end, its the matter of your system and taste.
For example, in my system both are silky and smooth with HQ records, but HD800 were more forgiving with some records I never even thought they can be. Not only because of that, but the music itself, never thought AKG will reveal some nasty parts of it and HD800 not, was kind of surprise, but there are more things than just that.
For example, AKG just better in one important department, transparency, where HD800 are more colored, if you will, they have their signature that tells you what Sennheisers think the sound must be like, no matter what you listen to it, they sound almost identical, portrait things itdentical and dont change much with differ records and many folks like that, so, for each his own.
My big issue with HD800 are their midbass and imagine, both make them less natural and less balanced compared to AKG, thats why, probably, you see many recabled HD800, I guess, I dont know, cos I only tried stock pair.
Anyway, like I said, for each his own, but the money you pay for Sennheisers are something differ that you pay for AKG, even new, thats where other things starting play a huge part for me, but I leave it there and dont want to trash this thread with it...