I would like to opine that the problem may not simply be one of brightness, though I agree that there ought to be more bottom range and that anything above 60% or so is practically useless. The major problem, which may or may not be endemic (can't compare to Paperwhite, don't have one ... yet), is that even at low levels the black font begins to wash out and take on a bluish hue. I suspect that a slightly warmer light temperature might have made text appear (at least psychologically) more "black". Or else it's an issue with how exactly the light is distributed. Since we are using eReaders to avoid backlight, this is still the lesser of evils, but somewhat disappointing nonetheless. I avoid reading my Glo in total darkness.
Color temperature brings up an interesting point I noticed. If your ambient light is LED or otherwise cool, the (illuminated) Glo looks quite pleasantly white. Under regular incandescent, it looks too blue. I wonder if the reverse is true on the Paperwhite.