After reviewing your order history, I see you purchased the set of 3880 refillable carts and ConeColor inks in Sept 2013, and it looks like you are using ConeColor inks with other printer models as well. I see you bought ConeColor ink for a 7800/9800, which uses the same inks as the 3880, with the exception of the magenta + light magenta. Is there any chance you may have refilled the 3880 carts with magenta ink from your 7800/9800 ConeColor ink set?
It can be very difficult to get a totally neutral BW print with color inks, as there is generally some sort of color cast. If you were able to get neutral BW prints from your 3880 ConeColor setup when it was first setup, but have a magenta cast now, then I wonder if you may be dealing with settled pigment, which can effect the density and color of your output, or you may benefit from using a new profile that is specific to your exact printer/ink/paper/computer setup. Do your regular color prints also have a magenta cast, or is the color cast only visible in BW prints?
Please let me know, thanks~ Dana