I have the Logitech Z906 and let me tell you, they are amazing. I have used headphones, speakers and amps and to me nothing compares to the Z906s for gameplay. If you are listening to music, then 2.1 or headphones+amp is the way to go, but for gaming and movies the Z906s are truly amazing.
I loved them from the day I bought them and can't say enough for how good they are. The bass completely shakes the room (and I mean this literally), the sound is crisp and the speakers feel sturdy. Being THX certified also helps haha
Good luck and I hope you do purchase these speakers so that you may experience the joys that I have