Digital BW, The Print

Darrin,

You may have one of your settings wrong. I get great prints using VM
inks, and Paul's curves and the 1280 printer No posterization at all. 
Are you using the Adobe 1998 color space as your working space? Using
all the inks? Heavyweight Matte media setting? Error diffusion on? File
in RGB Mode before printing? "Same as Source" in the final dialogue box?
All these settings matter. 

Post ALL your settings in ALL the dialogue boxes, and lets see if you
have one or more that aren't correct.

Jerry