If you are using the AD700, then you DO NOT need an amp. It is definitely unnecessary.
That is one of the great things about those, is you don't need an external device to power or improve it. It sounds perfectly fine on it's own, and you will find little if no improvement if you use it with one. Besides, I find playing games with an amp unnecessary as well. I'm more of the "plug and play" after all, it is just a game. Save the good stuff for your home rig, where you actually listen to music.