Boox's screen contrast is a bit lower than Kobo's, meaning that the background is perhaps up to 2% darker on a scale from 0% to 100%. Not much, and very readable still, but clearly noticeable.
Knowing there is only one manufacturer for the Vizplex screen, I wonder if any technical explanation is available. Since the screen is placed in front of the Wacom touchscreen, shouldn't be any difference.
... any technical savvy around?

It's probable due to the protective material placed over the actual display.

are you using the same font in each? same book file?

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.

the reason for the difference would be something in the code that drives the display. I posted in another thread that if you read the Boox with full refresh off you get a whiter area around the letters than the normal backround. So their blank state is not as "white" as it could be.

I think the contrast is a bit lower in the Boox because the text is dark gray, not black. Look at the status bar to see what black really is.

For those interested to see the screens of Boox and Kobo side by side I took two pictures in the same conditions, loaded into Photoshop and converted them to grayscale.

If you open the Info Window in Photoshop and hover the mouse inside the black rectangle added to each picture, you see grayscale ranging from 28 to 32% for Boox, and from 23 to 27% for Kobo.

I think mr. Dulin is right, and I also found that minor differences may occur even in the manufacturing process of different Vizplex batches.