[cristianm]

I used the same file (in fact I have the same book collection on both devices).

The difference is still there when I shut them down, aka blank screens. I've printed a rough calibration paper page with gray stripes going from levels of 1% to 14% grayscale in 1% increment, and I would say Boox has a background of about 9% level of gray and Kobo is closer to 7% (measured in natural light in a windowed room, otherwise these values may be lower - for example in direct sunlight).

There's absolutely no criticism in this, I'm perfectly happy with both devices ... just looking for a tehnical explanation if any.