No, because the treble is above the room transition frequency. Unless you are in a tiny room with no porous surfaces (the walls are made of glass or something), the difference in treble should be from whether you are listening directly on-axis or on a different angle...and if the speaker has un-even dispersion the differences in position will be exacerbated. I’ve never heard a speaker that’s bright that isn’t bright in every other room.
If you have an experience like the one you mentioned, and the angle you listened to the speakers didn't change, assuming the speaker is normally not considered a bright speaker, another possibility is the speaker has an existing peak at a frequency that was exacerbated by some reflections that grated your nerves and so it wasn't actually bright, rather you were just bothered by a high q resonant peak that was made worse.
If you have an experience like the one you mentioned, and the angle you listened to the speakers didn't change, assuming the speaker is normally not considered a bright speaker, another possibility is the speaker has an existing peak at a frequency that was exacerbated by some reflections that grated your nerves and so it wasn't actually bright, rather you were just bothered by a high q resonant peak that was made worse.