Thanks a lot for all the input.

I have been using the matrices from *)
and thought I was doing sRGB/D65-to-Lab or sRGB/D50-to-Lab.
But as I understand now, I was actually doing sRGB-to-Lab/D65 or /D50;
while all the RGB spaces like AdobeRGB or sRGB have their own fixed 
implicit reference white.

*) This is where the matrix used by babl and the Decompose plug-in is
    listed as "PAL/SECAM")

It all almost seemed to make sense.

Then I read the wiki on sRGB where it says that sRGB has an illuminant 
white point D65, an encoding ambient wp of D50, and a typical ambient wp 
of D50.

Then my head exploded and I decided to keep this strictly on need-to-know...

So, summing it all up, I am not doing anything wrong when I do the 
conversion from sRGB to Lab/D65?

- to be like Photoshop: D50
- to be "more right": D65

As to babl and the Decompose plug-in: They should be updated to use sRGB 

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