I used an Epson EQL5700 some years ago.
It made deep dark prints with a heavy layer of toner if the "toner save" modus was disabled.
It also had a software feature to determine the thickness of the toner.
Printing a transparency there is no light supposed to pass through the black portions of the print.
Trying to compensate by using double mounted transparencies works only with small boards. With larger size boards (160X100mm) both prints are not scaled 100% and don't match.
If you are looking for an accurate print try to find a print shop where they make advertising stuff. Take along your Eagle installation file and a six-pack of beer.
May be you get some help.
Boncuk