First thing to try is what the manufacturer is suggesting given the apparently unconventional design.
If that doesn't yield improvement keep in mind that treble perception is linked to bass - if bass is weak, treble will dominate and sound bright and if the bass is too much then treble sounds dead, all with no change to treble output. It's all a part of the whole.
The other thing to consider is that we tweak systems, not components. Given that, you might have tweaked performance to push up treble with your earlier speakers to get the sound you wanted which is causing you issues with a more neutral speaker. Assuming components are not the issue, try removing all tweaks to your system and starting from that baseline. I've found that many tweaks I have made in earlier configurations have been counterproductive as my system performance has increased.