It's sort of true though, the automatic room correction part does not do a lot above 1-2khz, probably because attempting room correction much above the room transition is a bit of a crapshoot with reflections, listening position etc. anyway, to my understanding. You can however manually apply filters in GLM in whatever frequency range your heart desires.
Yes, this is the answer. To my knowledge GLM corrects up to about 800Hz, to correspond with the range controlled by one of the hardware DIP switches on the back of the speaker (when the speaker is used in manual mode without GLM) - but for all I know GLM could in fact go up slightly higher to the 1-2kHz range.
Regardless, as
@JustJones says, there's the adjustable high shelf filter in the Sound Character Profiler, which is a target/preference curve tool that you can adjust after, and separately from, the room correction. And if you really want to get into it, you can also manually adjust - and add - filters to the room correction filter itself, almost without limit.
I have experimented with this latter feature myself, but I have found that my manual adjustments either made no audible difference or changed the sound for the worse.
The Sound Character Profiler, by contrast, is dead-easy to use, nondestructive in that it doesn't change the underlying room correction filtering and EQ, and produces clearly audible, non-jarring changes in the perceived tonal balance, which is exactly what it's designed to do.
It's a very common complaint with pretty much every automated room correction app - GLM, Audyssey, Dirac - that the corrected sound is sometimes perceived as lacking in bass. It does seem like some apps have some room for improvement in this regard, but GLM isn't one of them, and on the whole I think the problem is that a properly designed app will knock down the peaks but not boost the nulls (since doing so would just take more amp power and stress the drivers more to no audible benefit). So the overall bass energy is indeed reduced, albeit more linear.
But again, if you feel the bass is too attenuated and the high end too prominent, that's what something like the Sound Character Profiler is for. GLM limits you to +/- 3dB on those shelves, but that still enables you to alter the bass-treble balance by 6dB, which is considerable.
