It was scary listening to a few different waves through headphones. At certain frequencies, the perceived loudness dipped, despite the fact that the meter stood steady at -6 dB. So much for my hearing. I think I'll explore where my hearing's weak, and then keep that in mind when mixing. Sheesh.
How do you know that it's your hearing? Why couldn't your headphones have a frequency response all of their own? (They do, you know...)
And actually, it's worse than that. Most headphones form a complex resonant cavity with the ear, and this can dramatically alter the total response that you, as an individual, will experience from them. And your experience won't necessarily be the same as anybody else's with the same headphones. Open-backed ones are generally a lot better from this POV, but they are by no means blameless. It's one of the reasons that when you mix, you really shouldn't do it with headphones at all.
So don't write your hearing off yet on the basis of a swept sine test through headphones - you haven't really tested it at all.