I don't think the wireless component of the Mixamp 5.8 is what causes the hiss.
I think the hiss is caused by a less than perfect SnR, same as with devices like the DSS, and the 2013 mixamp. The wireless part of it works great and sounds, to my ears, undetectable and identical to wired devices. Perhaps when you turn the volume way up it becomes audible, but I'm not sure if that's because it's wireless or just because the amp is a bit noisy.
Edited by chicolom - 2/5/14 at 8:12pm