cholmondeley,
I have been using the BOSE X set for the past 6 months and it is superioir to any other NOISE REDUCTION headset out there. As for High or Low Impedence, that would depend on the A/C you are flying.
Someone mentioned that Low is only for Military A/C, not true. In our worldwide fleet (300+ aircraft) we are about 70% High and 30% Low impedence so check with your operator. Hi or Low does not affect in any way the performance of the headset.
Cheers,
OffshoreIgor