Okay I might have 3 suggestions here on stopping yellowing cases:
1) Have you tried cleaning them with Fantastic ORANGE cleaner and/Or Oxy Clean and bleach (for worst case)?... I have several older Amiga models with the off white cases and an old C=64C which has the same coloration thing going on.. They cleaned up very well and yellow was gone after I bathed them in a small tub of bleach (after they were removed from all electronics of course.
2) Stop SMOKING... If you smoke your case is more likely to yellow. I have seen that over a 15 year period for people bringing in older equipment to the dealership. We cleaned all that stuff up all of the time....
It may not be the "whiteness" of the old plastic as much as what's in the air of your home.
People will probably blast me for saying this but we had a 100% correlation to yellowing cases to people who smoked way back when I worked for a C= dealership.
We'd ask anyone bringing in a yellowed case for repair if they smoked and their answer was always unoquivacably "yes"... It personally grossed us out to think what that stuff was doing to people' lungs if their cases turned yellow.
3) If you smoke or don't smoke and you have a yellowed case try what I suggested in #1 it works for me everytime..
