Not doubting you, but what makes the W. 500s bad? Some people were downright waxing poetic about the things.
The center-pulls all fit with the Kool-Stops...the question remains whether the W. 750 is a better fit on the rear than the 610 or not. I'm guessing not, but I haven't really analyzed that aspect of things yet. One semi-cool bit about the 750/610 pair is that they're labeled "Raleigh" in blue, rather than the "Weinmann" in red like the 610 pair I have. Trivial, but I'm easily amused.