I own the Siberias, and they are most definitely a downgrade to the AD700s.
I haven't heard the 555s, but since they are essentially the 595s pre-mod, they are pretty on par with the AD700s from what I gather, if slightly a little worse.
Since you already own the 555s, and want a clear upgrade for pure gaming, I'd say go with the DT770 Pro 80s IF you game with Dolby Headphone or some other virtual surround, and don't mind a bassy headphone. Otherwise, perhaps the K701, though they essentially improve on everything the AD700s have to offer.
Like GodMode, I too am not really an audiophile, and mostly use my headphones for gaming, I have bought/tried plenty, including the A40s, Tritton Ax Pros, Siberias, AD700s, DT880s, 770 Pro 80s, K701, M50s, and PLENTY of others not so well known for gaming purposes. The DT770 Pros and K701 are VERY different from one another, but both excel in gaming in their own ways, which is why I'm completely happy with them and don't need another gaming can. Whenever I want pure enjoyment out of gaming with movie theater-like sound, the DT770 Pro 80s are the way to go. When I want to compete very seriously, I put on the K701s. Believe me when I say that this is a winning combo. If I seriously had to choose, I'd take the DT770 pros because of how fun they sound when playing games. I can still compete with them on a serious level, though others may find the bass too powerful for true competitive gaming. Still, they DO have the very best positioning and directional cues I have EVER heard for gaming.
GodMode, I think the K701s would suit your needs as an improvement over what you have now. I speak of them purely for gaming purposes. You ought to know how popular they are for music by now. If the price is too steep, the AD700s are pretty much the next best thing, for a fraction of the price.