The title page of the first edition of the מעשה רב (Ma'aseh Rav) , R. Yissachar Ber b"r Tanchum's important collection of some of the practices and positions of the Vilna Gaon has something interesting:
It's title in Blackletter (=the German alphabet of the time) reads Masse Rebi (and not the expected Raw or Rab). It's possible that there's some sort of Polish explanation for this (it was printed in Zolkiew, Galicia) but I put forth my conjecture that whomever it was that set the type (or wrote it or dictated it for setting) pronouncing the Hebrew רבי (rabbi) instead of רב (also rabbi), and doing it in the traditional Ashkenaz way as Rebbe.
Compare with the Warsaw 1858 edition:
Recognizing of course that there are many subtle variations that I can't reproduce with mere letters, and that in 1808 surely no one said it American-style like Reh-bee.