While I've never tried the SB G5, I used to have a STX when I was gaming a lot. The STX is good for most gaming head sets but is not really designed to drive the HD-650/600 properly IMO. The K7XX is a great headphone for the price but its still not in the same league as the HD-650 due to recessed mids and slightly grainier highs. It does have a better sound stage and better positional audio if playing games and its easier to drive properly compared to the HD-650. The genre of music that you like will also influence the choice between them, for classical, movies, and genres with a wide sound stage the K7XX would be my choice, for all else the HD-650, but only if it properly driven, to me that's my BH Crack or Ember.