Digital BW, The Print

Printing at 1440 I find Black Only and Photoshop-one-step-desaturated
black are very close, and match my monitor visually very closely.
However, at 6X9" the BO dots overpower my 900ei film's grain...whereas
the film grain becomes visible in the significantly smoother tones
produced by the Epson driver. 

Black Only usually seems inferior to CS2-managed Epson/UT7 from
scanned files, though the dots add zip to grainless digicam files. 

The blacks and overall color appear nearly identical (though the Epson
is obviously smoother and the BO is higher contrast), but on close
inspection there is sometimes a tiny amount of R(Y/M),in the Epson
smooth high greys, and sometimes Y in the highlights, barely enough to
discern on Moab Kayenta (ultrawhite like EEM). This is with UT7, not
Epson pigs.

Question: Which inks are being used by the Epson driver when I
directly print from a Photoshop-one-step-desaturated file?