Do you have Repro Desk on the 9400 server side, or do you just have to plot straight to the 9400?
We have used the Oce 9400/TDS800/TDS860 for 14 years now and *still* use an old HPGL/2 driver (specifically one for the DesignJet 750C). It works just fine.
Lately we have started to plot to PDF as a default. Repro Desk will handle the HPGL/2 files and the PDF files for us.
If you don't have Repro Desk, then I don't guess you could send PDF's to the 9400 (at least from AutoCAD).