Digital BW, The Print

Can I install only the MIS Eboni cart in place of the Matte 
Ultrachrome or do I need to replace the light black as well with the 
MIS K7 equivelant ?

I guess what I am saying is, in Black only print mode on matte paper 
using the Epson stock driver (Win XP), is the printer using ONLY the 
matte black cart or both the matte black AND the light black ?

Thanks