Yep. It sure is hard to discover. I don't think I finally figured out how to unlock the shelf filters manually until a few days of using GLM, after trying unsuccessfully to use the "Sound Character Profiler" menu.
Yes it's possible, but extremely counter-intuitive. For reference for others with Genelec GLM-enabled speakers, below shows how to achieve more control than the "Sound Character Profiler" menu itself will allow you.
At first when you open the Sound Character Profiler, you see all these preset curves, along with an option called "Manual". At first you may think, if you want to tune the Sound Character Profile with the most manual control available to you, then this "Manual" option must be it, right?!
Wrong.
View attachment 151822
Believe it or not, in order to gain full control of the target curve, you have to first select
"None" in "Sound Character Profiler":
View attachment 151821
Then, open up each speaker's filters window by clicking on it. Here, you can edit both High Shelf filters however you like. Here is an example of -40db combined treble rolloff from both filters, which of course sounds horrible, but is possible:
View attachment 151825
Whereas if you had used the "Sound Character Profiler" to edit this (even with the "Manual") option, it will appear disabled like this:
View attachment 151826
And as Amir pointed out, the "Manual" mode of the "Sound Character Profiler" only allows you a manual tuning up to a certain amount, and with limited granularity. Basically, all the "Sound Character Profiler" is doing is applying values automatically to all of your speaker's High Shelf filter slots (and disabling manual controls of them in each speaker's menu). But for some reason, the "Manual" mode of the "Sound Character Profiler" is significantly limited in the range and granularity of the parameters it allows, even though the filters are capable of much more. So to unlock the full potential, you need to set "Sound Character Profiler" mode to "None", and manually edit the high shelf filter parameters yourself as desired (for full control).
It makes no sense to me why they would design it this way. Terrible UX. But at least it is possible to get full control of the filters, but they sure do make it unintuitive. Just remember that in this case, you need to manually apply the same high shelf filter parameters to
every speaker individually, as it does not automatically mirror these parameters when "Sound Character Profiler" is set to "None" to free up them as manually controllable.