Phaser 6510 PDF grey text is too coarsely halftoned

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I've been exploring the printer's built-in promised postscript 3 support by using "lpr -l" in unix. This sends a raw postscript file direct from the workstation filesystem to the printer and the printer interprets the 2d drawing instructions. This so far reliably provides the highest quality.

Even this method cannot escape the inappropriate halftoning and postscript example files using setscreen and sethalftone functions of postscript cannot print correctly.

Some xerox printers have a screenon.ps and screenoff.ps pair of files but I can't find much information about how to use them even if they're available somewhere for the Phaser 6510.

If anyone knows how to unlock postscript 1, 2 and 3 screen and halftoning for this printer, please let me know. I bought it for the postscript feature to be sure I can get the right output and I want to use it.

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The firefox-based pdf viewer seems to have caused the selection of a grey-level instead of black, but something else is responsible for coarse halftone lines instead of a nice fine stipple.

Discovered by printing something with light-grey rectangles.

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It seems printing from firefox's PDF viewer (or ubuntu firefox plugin for PDFs perhaps) is involved in the causal chain. When printing from ubuntu's standalone PDF viewer the result is very good.

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When I print a PDF which is to come out with grey text, the halftoning is a very coarse diagonal stripe pattern which makes the text somewhat unreadable, but even at 600x600 dpi I was expecting a very fine halftoning.

Does anyone know why this is happening even with the official Xerox driver for Linux?

And what can be done about it?

I'm a highly technical user being a software engineer -- including Linux appliance development -- for almost 20 years professionally and longer as a generic computer nerd so the most advanced suggestions are also welcome.

A slightly out-of-focus example is here:

(Actually this should have printed in black but it came grey instead - I'll try to tackle that another day)