HD 600 on a MixAmp:
Mix--fully on "Game"
Volume--about 1 o'clock
Tested on Mass Effect 3 (campaign), running around Citadel and a short, indoors side mission.
Dialogue was fine; speech was perfectly intelligible, always. This included when I hit the nightclub on Citadel; bass was nice and thumping but I could still make out all of the speech with dialogue trees, and most of it when eavesdropping on NPCs.
Jumping mass effect relays produced a satisfying "THOOM", but I did hear a crackle just prior to making the jump--is this part of the intended SFX?
Positional audio was okay. I could detect well enough when shots were fired 'somewhere' behind me, and during dialogue sequences I think the 6 o'clock was a bit clearer than on my DT880. Could more or less tell when people were further away, but since this isn't a sneaky game, positional audio only comes into play in all-out firefights.
Combat took place in the same type of environment as the talky bits, so there was no audible difference in the way stuff sounded (e.g. in an echo-y chamber vs in a small room).
It was overall an okay experience, nothing to complain or write home about. 600 was a bit clampy, but it is 3 days new...