Frood is probably right.
Another idea: the noise could be actually (or partially) caused by reflection. Is it still so noisy if you disable reflectivity?
Thanks, I just tried to disable ALL reflections, but no luck, still a bit noisy. Good idea though. Perhaps I should just accept that the scene is a bit hard to render because the mail source of light is refracted/transluced(?) through a material. I made another attempt with a translucent material on a "thick" plastic. (2mm). This also has white absorbtion set to 10mm, and white scattering. I appreciate that the surface reflections show up clearly when using translucency. On the left side of the green speaker on top for example. I think this is perhaps the most realistic way to do it. 279 passes this time, at 1k rez.
(I'm *really* hoping for a native region render in the Corona VFB in future releases! :D)