Digital BW, The Print

I'm having serious trouble getting the ultratones to work properly on
glossy papers with my 2100. I'm not 100% sure, but it seems to be the
light black ink interfering, making it impossible to cool down the
shadow areas to anything less than a really brownish gray. The problem
is also there to a much lesser extent on matte papers, but the prints I
get still appear totally neutral to my eyes at least, so there it's not
a problem.

Here's a scan of a 21+100 step wedge on both matte and glossy papers
where I toned the image with an extreme amount of magenta, which in
theory should make the entire curve blue, but you'll see that from about
85% the curve starts turning brown. 

http://daniel.staver.no/img/ultratones2100.jpg

Any suggestions on what I could do to solve this problem? I don't think
any kind of curve adjustment will solve this, so I think the solution
must be in changing the light black or one of the other inks somehow.
I've been wondering whether I could mix a more neutral light black and
keep a cartridge of that available. It's not a big deal to change
cartridges on the 2100 so I would consider that a practical solution to
the problem. What would be the best way to mix a neutral ink? I've never
mixed my own inks before, so I'm not really sure where to start. Would a
combination of MIS Light Black and some UT Cyan position ink work?

--
Daniel Staver
http://daniel.staver.no