I get hiss on my SE846s with the Mojo (and on an RSA Intruder on low-gain, and an ALO Int+ OE on low gain). On the Mojo and ALO the hiss is not volume-dependent, on the RSA it is.
Someone suggested that, since no one else seemed to have reported any hiss on the RSA or the Mojo, it was the headphone cable causing it (acting as an antenna).
So, since I had an easy week, I took my portable gear into one of our labs to test it a bit further. The lab in question deals with state-of-the-art satellite and radio communications and is equipped with a chamber that is electrically silent. No signals get in or out. Any electrical or RF noise in that chamber can only come from what's taken inside it.
I got exactly the same hiss results with my gear in that chamber as I did sitting outside it, or sitting in my living room. So if my cables are the issue, they're picking up noise from the electronics being tested, i.e. the amplifiers (since this hiss occurs regardless of whether there is anything else present or connected).
Now, to be sure, on the Mojo the hiss is incredibly quiet. So quiet that in a brief quite patch in music it might not even register, but it's definitely there. Oddly enough, sometimes, the hiss would be louder than others (this seems to be new thing, as of mid-week) - and in those cases touching either of the volume buttons would quiet it back down again.
The hiss is completely inaudible with any of my non-IEM headphones.
I don't view what I am hearing as a problem. It doesn't intrude on the music. With the Mojo it's the lowest level of hiss I've heard on the SE846 from a portable DAC/amplifier. But I know I'm not hearing things and now I know it's not the headphone cable acting as an antenna (unless the sources themselves are the source of any radiated noise).