I think that FPS benefit the most from using headphones. Since the view on the screen is coming "from" your head in a FPS, the audio in the game matches perfectly with the "in" your head sound of headphones. When someone starts blasting on me, it's nice to be able to use the precision of the sound-stage to hear exactly where they are shooting from. You can almost play with your eyes closed.
So I just did an A/B comparison between my tube amp and my soundcards line-out with HALO 2. The biggest advantage of the tube amp was the much more enveloping sound-stage. When using the SS line-out of my soundcard, the sound-stage was primarily to the front only.
Things like water falls, nature sounds, rain drops, etc really fit into the overall picture a lot better with my amp. I felt more immersed in the experience with tubes.
The battle rifle was a bit cleaner sounding from the soundcard, but it sounded kind of sterile in comparison. The tubes made it sound a lot more powerful. I'm sure this is due to the distortion of tubes, but I think the explosions sound better due to the slightly bassy sound. Even though the bass is augmented, it still has snap to which is quite noticeable on the bigger rifles. Makes it sound more punchy. I also get fatigued less when using my tube amp when I play for a few hours straight.
Whenever grenades land close to me I can hear them perfectly and start to move away instantly. I can hear the plasma grenade's sizzle even while still flying in the air towards me. This detail is less pronounced when just using my soundcard. Footsteps seem to come through better with my tube amp also. A big help when people try
sneak up behind you!
So overall I prefer using my tube amp whenever I game; I enjoy the experience more with this amp. Especially with FPS and it helps me a lot in HALO 2 live battles. .