I have 1Z and IE800 too. To me there is no special roll off in the trebles in that combination. I like the IE800, but it has a little bit emphasised and at the same time kind of thin sounding trebles, independent from source. And I don't feel about the 1Z sounding thick at all. It just sounds natural to my ears, not dap-like, not digital- like, whereas the NW-WM1A which I also own sometimes tend to sound a little bit pierching in the highs. Many people seem to like emphasised trebles. They let us imagine more details than there are. But, from my experience, very much depends on customization to a special sound signature. When I use the same gear for a longer time it's sound sig seems to become kind of reference for my brain. I tend to percieve differences to that reference sound sig often as totally wrong at the first moment. There are many pitfalls with our hearings I believe, or at least with my hearing..lol.