ANR technonology only attenuates low frequencies, whereas high frequencies (hissing) are passively blocked by mechanical means. The Sennheiser HMEC 400 is very good at both, but the passive attenuation is achieved by squeezing the earcups uncomfortably hard against your head. Bose X are much more comfortable, but at the expense of letting high frequencies through. Some people work around this deficiency by wearing foam earplugs inside the earcups, and turning the volume up so that they can still hear the important stuff. Personally, I'd rather just wear the earplugs of my Clarity Aloft headset, and not have anything clamping my head.