OpenGL inaccuracies and black levels

Here is an sRGB file consisting of 0,0,0 background, with five overlaid squares at one level increments, from 1 to 5.
On my Eizo CG246, calibrated with Eizo ColorNavigator, all five squares are discernible against the black background, as long as Graphics Processor > Advanced is set to "Basic" mode. This means that the color management logic (the conversion to the display profile) is performed in the CPU.
In "Normal" and "Advanced" modes, color management is shifted to the GPU. With these settings, black levels disappear. It's not dramatic, but it's there. The difference can be illustrated in a screenshot, as long as the display profile is assigned and the screenshot is pasted in the Basic mode. Here's how it looks straight up (should be viewed against a dark background):
Here I put a Curves layer on top to exaggerate:
And here I read out the numbers from this exaggerated version:
So far it seems this can not be reproduced on another monitor I have, an NEC P232W calibrated with Spectraview II. It also seemed a bit more pronounced on a different Eizo I no longer use, an S2243W, calibrated with Eizo EasyPix.
This suggests that the problem is in the interaction between the display profile and the Open GL engine that does the conversion. I think this is related to the ProPhoto cyan banding issue previously reported, because that also seems to behave differently between these two systems (I'll do some more testing on that).
In all cases and all scenarios, all irregularities disappear with Open GL in the "Basic" setting.

You neglected to include your test image with black and levels (1,1,1) through (5,5,5), but no matter, I have the one I made...
http://Noel.ProDigitalSoftware.com/ForumPosts/DarkGrayColorLevels16Bit.png
I've found that here I'm not seeing a visible difference between GPU and CPU color-management with the image in the sRGB color space.  In other words, no crush.  With the image in the ProPhoto RGB color space, some slight crush was apparent, along with color shifts almost exclusively toward red.
However, that's not to say there are no differences between the two.  They're just more subtle than what you're seeing.  What I did to test was this:
Open the image I linked to above using Advanced mode OpenGL.
Float it in a window.
Screen grab it.
Pick it up and start to drag it.
While still dragging, screen grab it again.
Overlay the two images, and pixel align them.
Set the top image to "Difference" blending.
Add a couple of Curves layers over the top to greatly enhance the differences to make them more visible.
Since my test image has a dark grayscale gradient expressed in 16 bits/channel, it's not only testing for accuracy at the visible level, but also for very subtle changes.  Lo and behold, changes are revealed.  Note:
Enhanced differences between GPU and CPU rendering of the image in the sRGB color space:
Assigning the ProPhoto RGB color space to my test image, then comparing GPU vs. CPU rendering...
-Noel

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