Perfect Solution That Will Run Well For Everyone
is to actually lit the places where it matters - ie where player will have to go or will have to look at, instead of relying on a dim ambient that only works with high gamma settings.
Doesn't help that some of your textures are very dark, like peaking at 40 and cutting off at 70 (out 0-255 obviously) and you can only go down from those values since no overbrights.
Btw my monitor gamma is brighter than SRGB (I can distinguish the lowest grades, even 1-2-3 depending on the view angle
http://www.lagom.nl/lcd-test/black.php) and I found some places too dark as well.