akg 7xx, great sound stage and audio positional is great too. However certain pitch is sharp, probably the treble is high.
senn HD6xx has narrower sound stage compare to akg7xx. Still there are people mentioning the HD6xx has better imaging than the 7xx. As for the sound, it is more neutral.
P/S both need AMP/Dac. AKG 7xx is easier to drive comparing to HD6xx