Silly me, I forgot about pH. That being said however, why does a fixer pH difference cause color shift? I've heard of that happening with developer, but not with fixer.
When you say higher contrast how much higher are we talking? Do you have contrast specs for e.g. Ektar 100 and Kodak 5207?
Because when I look at the sensitometric curves for these two, I'm seeing a Dmax-Dmin of about 2.8 and 2.6 respectively; for about 14 stops of dynamic range each. I could be wrong though.