I don't use the contrast feature of the 650 but I rarely have poorly scanned pdf's to read from either. In terms of fonts, I chose to use caecilia. I went with:
Caecilia Bold = Standard
Caecilia Heavy = Bold
Caecilia Bold Italic = Italic
Caecilia Heavy Italic = Bold Italic
You can see below is what that looked like on my 505 (it looks even better on the 650):
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