Its not unusual for different plotters to provide slightly different output from the same CTB. We used to have a 5SiMX, a Kyocera, and several networked Xerox printers all for 11 x 17 output and they all produced varying levels of screening. You can fiddle with the printer settings for a week but you *never* get it just right. When different printers are that far off from each other its usually faster to create a separate CTB. Its not that big of a hurdle, just set up different named page set-ups for printing to each e.g. we have separate CTBs for our current crop of 11 x 17's as well as our Oce and HP color.