I've experimented for a while now and there are 3 ways to get the compositing done in Photoshop. They all use Linear Dodge (Add) as blending mode.1) Export straight OpenEXR and correct gamma in Photoshop (32 bit)
If you export the passes as OpenEXR directly from Blender, you need to adjust each layer/pass by a gamma of 0,4545 (1/2.2). This looks something like this ...2) Export gamma corrected OpenEXR from Blender (32 bit)
If you place a Gamma node set to 2.2 between each of the passes and the Output node, you don't need to compensate for it in Photoshop. That looks something like this ...3) Export TIFF and work in a linearized color space within Photoshop (16 bit)
If you prefer to wotk in 16 bit you need to convert your imported passes to a linearized color space with a gamma of 1. Of course you have to save it in this color space as well if you want to preserve the layers. I've uploaded the ICC file in case you should need it.http://www.andreasresch.at/upload/Linear%20sRGB.icc
I will see which one of this methods I will use - probably 2 or 3.