I seem to have solved the problem with the harsh-sounding treble. This in no way will suit everyone but for me it's perfect. I just put some light, thin fabric on the inside of the earcups (see picture). This makes the treble sound a-lot less harsh, but it does also make the bass sound a bit flabby, which I don't mind at all. Overall, the detail is lost a little bit, but the general sound is slightly more laid-back.
Like I said, this mod certainly won't interest everyone but if you are getting annoyed with the harshness in the upper registers you can easily give this a go.
Also, interestingly, I feel like this mod solves the issue I had with getting the positioning quite right. I would always be moving these around on my head trying to get the best possible sound, but now it seems like the sound is the same however I wear them. Somehow this mod must have messed with the whole S-logic thing ha. Not that I mind at all.