I've played with diffusion before and not really liked it, but diffusion is a difficult topic because (the way I see it) it has four broad effects depending on the size of the diffusion.
The largest size of diffusion spreads light across the whole frame, reducing contrast. I'm not really a fan of this one.
The smallest size spreads light into neighbouring pixels and essentially is the same as blurring the image.
The medium-large size spreads light between objects in the frame, creating halos and a strange appearance I'm not fond of.
The medium-small size spreads light far enough that it softens texture in the image. This is why all the Hollywood actresses wanted diffusion because it hid any skin imperfections. Apparently Cooke lenses all have this kind of diffusion in them but only for the frequencies of light around skin-tones - potentially a key part of the "Cooke Look".
Going back to your previous comment:
This has stuck in my head and after re-watching the Gawx videos I suspect this might be contributing to it.
I might have to do some investigation on this.
Also, now I have the anamorphic adapter and my new wide angle adapter, I'll step things up in terms of making things look a lot dirtier. Anything in particular you think I should test? (apart from having people in the image, which I think will end up being me).