PRS+ doesn't really affect EPUB viewing much. It allows you to change CSS files right from the reader, but that's it.
Anyway, try to fully restart your reader.
Kindle3 uses "bolder" sans-serif fonts, it makes them look crisper. In general sans-serif fonts are considered to look better, but be worse than serif fonts as far as reading goes.
Technically you could use amazon's fonts on your Sony, the problem is how to obtain them legally.
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