I don't think changing the volume affects it at all. It's there just before or after audio input. On my Mac it stops about a second after input while on the iPad around 10 seconds after audio input. It's not bothersome and hardly audible to my ears with these. I've owned Sennheiser's Kleer headphones for a while and the hiss was much more prominent there. More audible on mm450-x, too, I'd say. I the hiss on these doesn't bother me. I don't think it's there with quiet music at all until you go right down - to the point were the sound itself is hardly audible. Sennheisers were much worse.
If they'd have fully sorted the headband feedback issue and perhaps added AC these'd be pretty close to perfect. There are always tradeoffs, unfortunately.