In our test yesterday I played with my Senn's 595, stereo. I used a '2 RCA to 1 stereo' adapter connected to the XBOX HD cable. When we tested SS I had my eyes closed and could fairly well tell where sound was coming from. However, having said all this, when you are in full action fighting grunts I am not so sure if I would be able to tell. I guess this also depends on the player.
Last week I played with my Senn's plugged into the DH of my HK AVR 635. It sounded fantastic but I do not think it would help me better in orientation.
So, for me personally, my grand conclusions are:
1) Game with headphones. Just the emersion is worth it
2) Play with GOOD headphones. I played with a cheap headset, a reasonably noise-cancelling Sennheiser, iPod earphones and the 595. Not surprisingly, the 595 -by far- outperforms the others.
3) The JVC 5.1 adapter does not seem to be worth it if you use it for gaming only. Reviews are very positive for movies. Volume control of the JVC would be nice although the default volume for me is just fine.
4) The TRITTONs are a very good deal for all that you get (optical input, volume control, somewhat better SS). If it is truly for gaming only, and perhaps for not more than 1-2 hour game session at a time, I would recommend those.
5) I do recommend the 595 for those who game for long periods at a time, and for those who wish to use the headset for more than just gaming (although the 555 seem also "good enough" for gaming).
6) If you have a receiver with Dolby Headphone, use it
Thanks to all who contributed!