Canon Multipass F20 Hi-Res Problem

I've been able to get printing based on the the Debian distribution
of Ghostscript 6.53 on my Linux box (kernel 2.4.26) to my Canon
Multipass F20 working in both black & white and color modes at 600
dpi, using the cbjc800.ppd and bj8pa06n.upp files - albeit I'm doing
it using Samba with the printer connected to a PC running Windows XP.

My problem occurs when trying to use 1200x1200dpi resolution. (I know
the printer basically can handle this, because my test file (a
mapquest map of my neighborhood) printed from a Windows XP machine
using "High" resolution color printing look OK, showing much less
staircasing that what is printed from my Linux box.)

Whether I add "-r1200x1200" to the gs command line, or use
@bj8pp12f.upp (as supplied with the distribution), the result is the
same; I get one very grainy appearing page showing the upper left
quarter of my test file. I don't know if more would print because I
only put one sheet in the printer.

I just know I'm overlooking something incredibly obvious. The
appearance of the output just reeks of leaving out something that
squeezes 1200 dots into the same space that 600 dots would go. I hope
the answer isn't along the lines of "ImagingBBox" pm the gs command
line, because gs 6.53 seems to complain about that.