Sold my Z5 compact as I can't bear the hissing noise.
Although it is a bit late, I would like to share my observation.
The noise is significant in high sensitivity headphones or balance armature headphones
It is horrible with ATH-IM02, but can hardly be heard with IM50 or E40
The noise severely affects quiet and single tone sounds, such as the piano track below
https://www.youtube.com/watch?v=hNbbLDy94yg
There are a lot of Z5 series devices that have the headphone jack broken within a few months, including mine
I guess it is a problem in hardware design, and I guess Sony have changed the design in the new headphone jack parts
The problem is worsen with the new headphone jack.
Finally, there are a few temporary solution.
First, turn DSEE HX on. It reduces the noise by ~80%
But it means that you can't use other audio effect.
Second, if you have bought Poweramp, install beta version.
Go to setting > Sound > Output, turn on Hi-Res output
the noise reduction is close to DSEE, but you can still change audio effects