The MLG forum's resident audiophile weighed in on headphones
He's not a huge fan of the HD 555s but he did have this to say:
These are easy to drive headphones, and will work with anything that has a headphone jack.
His opinion is that headphones like the HD 580 are much better for the price range. Here's my problem with that statement. The 580s are a little bit higher priced, but he also claims that they will need an amp for things like an Ipod (though he says that TVs and stereos should power them fine). Well, buying an amp immediately vaults them up into a different price category, making them a completely separate (and much more expensive) proposition than 555s.
Regarding stereo connections: I'd agree it's probably a better idea, but my understanding was that you were looking for a speaker system, so why would you have a stereo receiver/amp just lying around? If you do, go for it. On the other hand, the TV is right there and he's saying that it should power them just fine. Given his background, I'd take his word for it. There's a diagram for the common sense impaired at the bottom of his post, if you're curious how you should be setting things up (lol).
Regarding team chat: the quote about Final Boss hearing their teammates just fine was because they play in tournaments. Their teammates sit right next to them and the open nature of the headphones means that ambient sound can get in. When OGRE 2 says something, Walshy's no more than 5-10 feet away, so he can hear it in spite of his headphones. I regret to inform you that I will not be sitting 5-10 feet away when we play Halo 3, so I stick to my advice. If you want to use them for online play, set team chat to speakers only (which will output my sweet, sweet voice to your headphones), then figure out a comfortable way of fixing your mic in place.