Holy crap how can you listen that loud? I top out around 1 or maybe 2 on gain 1 for my W4Rs. Maybe mine is louder? I'm one of the few who hears the hiss pretty plainly.
My first thought was that you were listening at a higher volume than I do. The impedance in ohms is pretty close between the Westone and the Heir. The gain or volume is higher than I use. So, it might be the output of your source or the level of the recording you are listening to. Without knowing these variables first hand I am not sure if you listen at a higher level than I do or not. Intuitively, the higher the level of the volume pot the greater the possibility of amplifying noise or hiss. (Please protect your hearing.)