It is possible that the cancellation is not perfect and there are still pressure waves hitting your ear at frequencies outside the range of human hearing. Or it

It is possible that the cancellation is not perfect and there are still pressure waves hitting your ear at frequencies outside the range of human hearing. Or it's also possible that just having the cans over your ears is causing fatigue, separate from any actual sound pressure. As far as a DC offset, that doesn't exist in actual sound pressure waves. That's an artifact of discrete representation of sound as an electrical signal. When a DC offset is connected to a speaker you don't actually get any sound pressure, because the sound pressure comes from the cone's movement pushing air (resulting in pressure waves in air). A DC offset just means that the speaker is being held still at some offset from it's "zero" or resting position. But the speaker is still being held still, un-moving, so no pressure waves are created. The "pop" you sometimes hear when a DC offset is present is the transition from a neutral signal (no offset) to the DC offset. The immediate change in the signal causes the speaker to move as fast as physically possible to get to the new position, creating a short, often very strong pressure wave (the pop). But once the speaker is at the offset it doesn't move unless the signal changes. As you say, there is no "constant pressure" in acoustics. Or more accurately, it's typically not a factor in sound reproduction. Technically we are all subject to the constant atmospheric pressure. But it's not the pressure that causes acoustic phenomena, it's the rapid fluctuations in pressure. It's even possible that the headphones are forming a seal around your ear and subjecting your ear to varying pressures as you move and the headphones shift on your head (too slowly to be perceived as sound).