Digital BW, The Print

I am scheming on doing a small promo piece that's very large, 
and very cheap. I'm outputting a digital file directly to this giant 
"xerox machine" that's used for copying blueprints. Page size will 
be about 24x36". It's like a very low-tech 
magazine/diary/travelogue thing.

Question: I sent a file to it yesterday that started with a tiff file 
greyscale, converted to Bitmap, and "Diffusion" type. The screen 
representation of diffusion seemed much tighter dots than the 
Halftone choice. But today when I saw the proof, the 
shortcomings of the machine showed through -- there were 
these "toner line" inconsistencies. My thought is that if I could 
"coarse up" the line screen, it might compensate for the toner 
lines.

Does anyone know what effect changing those Halftone settings 
has? Does anyone know the roughest possible setting?

Thanks in advance.