han810,
Speaking from experience with a Xerox 8830, the supplied Xerox drivers didn't work - we wound up using HP 755 CM drivers within AutoCAD to get the plotter to produce what we needed. You may try HP's 750, 755, 1055, (etc) and see if that gets you what you need. Also, check your CTB file setup, and try playing with the screening, dithering, and grayscale settings. Also make sure within the printer properties that this particular option is either on/available whatever.
HTH
Todd