I noticed a difference as soon as I turned the treble down. But then they pretty much sounded like the speakers I currently have!
That what I expected. You are used to the sound of speakers that fall in line with what you currently have.
I suspect that you are more sensitive to sibilance because you are not used to the normal sound of the way an s is enunciated and are simply not used to the extra detail that a truely neutral speaker delivers.
I also agree with Dr. House. If you go to a speaker that delivers more in both the top and bottom end, you will perceive it as more balanced.
You have the M3s for a month. I would take advantage of that. Find a good Audio store close to you and listen to a bunch of different speakers. Come back to the M3 and compare the same material on the M3. I think at some point, you will find a speaker that delivers more of the detail you like with a more refined upper end that does not sound bright or sibilant to you.