I don't use an Epson, but on my HP9800 I go into the print dialog box where there are controls to increase density and bump up individual colors. I output all halftone work through Ghostscript. You might want to noodle around in the printer dialog box, maybe looking at advanced features if there are any, and make appropriate adjustments. I get decently dark film from there, and Ghostscript has no mechanism for increasing ink density like some of the commercial RIPs do.
Just a suggestion.