I'm not so sure it's going to resolve a peak in the treble. The peak will still be there and adding more even harmonics to what is up there may not make much of any difference. For example if you have a peak at 7 kHz, the second harmonic is at 14 kHz, the rest are well beyond what we can hear. Why would adding more to the second harmonic at 14 kHz tame that peak? I think the tube sound adds warm (even) harmonics (timbre) to musical notes of far lower pitches, which is a different situation. Perhaps the tube sound of a traditional pure tube amp with an output transformer adds to the sound, but guitar players want different things like soft overloading and saturated output transformer sounds that we definitely don't want. Wouldn't you say that depending on the design a hybrid adds some measure of the tube sound?
I think it's more that it adds more warmth (bass), so the treble doesn't stick out as much.