My data points now owning 305p and 306p.
My 305p are connected to a Fiio K5 Pro, I keep the knobs on the back of the speakers at 6/10, and there is no hiss. If I turn the knobs up to 10/10, and put my ear right next to the drivers, I can detect a very quiet hiss. I can not hear it in my normal listening position which is about 3 feet from the monitors (computer setup).
My 306p are connected to my Onkyo receiver in my home theater setup. There is a horrible hiss all the time, really more of a buzz than a hiss I'd say. It doesn't affect my enjoyment of the speakers because I don't notice it when watching TV or movies, but when the TV is off I can hear it if I'm in the room. The previous set of speakers I had in that setup were passive Bose speakers from 20 years ago, and there was no hiss, so I tend to not think it is anything further up the audio chain (fwiw I am using the same brand and model of cables).
This would lead me to conclude either 1) the 305s don't suffer from this issue and 306s do, or, it is a unit to unit variation.
For my purposes, I got the hiss in the place it doesn't bother me, cause if my computer speakers were hissing all day it would drive me insane.
Otherwise, love both sets of monitors. Truly opened my eyes to the world of monitors.