adding an emitive material cancels the sun light

I have a scene with a HDRI and a sun light.
As soon as I add an emitive material, even very small and with low light (what could be a table lamp inside the building), the sun light is very much eclipsed and doesnt shine strong anymore on the buildings.
Is there something to tweak in the settings to solve that?

I tried to make the scene clear and simple with a cube (building) and another cube (with an emitive face).
If you set the emitive material to 0, the sun light and hdri comes back.

Blender File:
https://ufile.io/908fe (click on Free download)

a) Clamp values < 1 are hardly ever a good idea.
b) I don’t see the effect you described in your test file. With or without the emissive material, there is no visible difference in sun and HDR illumination.

What Blender version?
Are you rendering on CPU or GPU? If it is the latter: CUDA or OpenCL?

thanks for your answer
a) pertinent remark about the Clamp value, I put it to 10 and suddenly the problem was gone! ( I had searched for 3 or 4 hours before…)
b) With the file as it is, clamp was 0,96 the problem / effect was back there. go to material, put the emitive material strength from 0,1 to 0 and you shall see the scene lighting up.

Config: Blender 2.77, CPU render.