Yeah well, I can't really give you a technical/smart answer for this, mainly because it's a problem engine / renderer side.
What I can tell you is the sollution, although not a very good one because basically you'll be using only 30% of the actual color space, but make the difuse darker, a lot darker.
At the moment on c3 we are sticking with a mean around 55, and a dif color in the mtl editor of 170 170 170 or so.
Edit: Or, what you could do, is use a dif color of 128 128 128 (or lower, whatever just use a low value), that way you can increase the brightness of your dif texture, and get more color range from your texture, I would try this!
