Digital BW, The Print

I'm trying to do duotone and tritones on my Epson 1270.  Though the 
1270 is great with color WYSIWYG, in tritone it's not even in the 
ballpark with duotones and tritones.  And I'm getting strange color 
shifts even within the color that is reproduced.  The result is the 
same whether I print the file as a multitioned psd, or convert to RGB 
and print that way. Any suggestions would be appreciated.