AKG K712 for gaming. Yup, absolutely love it. It shatters the X1 as a competitive headphone and I think it has just enough of a fun factor still. BF3 and L4D2 were fantastic with it.
It could not replace the X1 for me because it's strengths, accuracy, timbre, realism and thin, analytical nature just do not work for me with certain songs or games (Heavy-handed action FPS' like Halo, Crysis, COD single player, games where you go around blasting things). I highly prefer the X1 out of my mixamp for console gaming, but I might need an amp there because the K712 loses some of the magic from my PC gaming. With PC gaming the SB Z is more than enough.
No magic can change the SQ of the Mixamp to Soundblaster Z levels. We need better devices.