Yes the hiss is at constant level, it does not change if I change the volume, gain, bias or any combination of the 3. But it sounds like it's there whenever the amp is active. The hiss goes away after a click which sounds like a relay and the amp goes in sleep or inactive state. I did not notice the hiss when I with the demo unit because I used my headphones (aeon, sine). I only tried NT6 briefly and I was listening at a louder level. However, the hiss is very obvious as soon as I tried my fitear 333. when I went back to NT6 the hiss is softer but I can still sense it because now I'm aware of it. Now I can only use my NT6 using the single ended output. But for 333 even the single-ended still has a slightly noticeable hiss. Of course, with aeons and other cans it's not a problem and the sound is just amazing. Which is the primary reason why I got the QP2R in the first place. But still at this price level and with this competition in the market I did not expect this level of hiss on my ciems. Even the older AK240SS don't have any hiss and dead quiet. Plus it can also drive the aeons just fine though it's quite up to it's limit.