Steve, is your Mac on the network? We plot to an ancient HP455ca over a network from G5's . To do this I installed GIMP drivers for our plotter and create a printer on the Mac with a manual IP address for the plotter and selecting the plotter model for the driver setup. We then either use the plot command direct to the plotter or, better, save to a spool folder and use grphisofts Plotflow for OS X to control trnasmission to the plotter.
This has produced excellent results with good lineweights and scale using AC 9 and Plotmaker