A few months ago a guitar single guy who I've known many years was playing at the venue where I work most of the time. I was working in the other room.
I took a break and listened to him for awhile. He is quite talented, but the the highs were excruciating and there was a lot of high-end feedback (squealing). When he took a break, I asked him if he got the feedback under control and he asked "What feedback?" He couldn't even hear it. He asked me to help with the e.q., which I did, and then he sounded great.
He told me he really couldn't hear highs much any more! DUUUH!
I think I'm starting to experience a small degree of high-end loss, but it seems to be only after an hour or so of playing. When I start out, everything is fine, so I just have to trust myself and NOT make changes during the night.