QuadTone Rip w/PS PC3 - What's wrong with my picture?

I'm printing a step wedge in Photoshop CS3 on an Epson 3800 using the QTrip driver. I must be doing something wrong because though the tablet prints just fine on the page (Pictorico White Film) the printer still covers the rest of the sheet with very-very-very light grey (just off paper white) ink.

This just seems to be happening with the step wedge and I can't figure out what I'm doing wrong.

I open the step wedge (tif) in PS CS3, select "print", then choose "negative" and "emulsion-side down". That's about it. I know the image isn't as big as the whole sheet.

Any clues?

** Update **

This seems to be happening (with this particular image - the step wedge) only when I use photoshop's "negative" option within the printer dialogue box. I re-printed the step-wedge with an "invert" adjustment layer and chose "emulsion-side down" from the printer dialogue, and presto! no light-greyish coat on the rest of the sheet.

I'm still baffled. I'd love to hear anyone's thoughts if they can contribute.