I just got a bit more time to play around QP2R. After hearing hiss with my ciems, I tried to find out if I can nail the source. It looks like the way the amplifier is designed, it has some sort of warm up period coming from off to idle state for a long time. The hiss level is very noticeable when you start to play music, but it appears to go down a bit after it is playing for more than a minute. If you pause for more than 30 sec. the amp goes to idle state and complete silence. hitting the play button and the hiss goes back to that noticeable level. On my NT6, the hiss goes away completely or is barely noticeable after more than 1 minute the music is playing. On 333 the hiss goes down but I can still perceived the background is not completely silent.