Salam aleikum,
The Masoretes were a group of rabbis from Tiberias who begin the work of editing the Hebrew text in 6th century. You should ask yourself: why they decide to correct the text of the Torah exactly in the time of Mohammed and not earlier ?
For me it is obvious, when Mohammed (pbuh) came to Medina in 610 AD, the Jews recognized him as the promised Prophet from Torah, but they were disappointed that he is an Arab, since they were hoping that he will arise among them as an Israeli. And so they alarmed other rabbi in Palestine and Sham to change the scripture (it is however only my personal opinion)
By saying "the pre-Islamic Torah would have not made any modification" I meant of course the matter of changing the names Ahmad and Mohammad (pbuh), but only if we assume that they didn't really know that this prophet will be an Arab, because if we assume that indeed they knew he will be an Arab then it is possible that they were trying to hide at least his nativity, but left his name living in the hope that he will one among them. At this moment one can only speculate what exactly happened, only Almighty Allah know.
From ancient rabbinical quotations it is obvious that the earlier text was different from the Masoretic one, buy wait, now, after 60 years of editing, they finally presented to the world this crap know as The Great Isaiah Scroll (1QIsaa). For example, take a look at the text of Isaiah 42:1 given by Rabbi Ibn Ezra and compare it with the Masoretic one:
Rabbi Ibn Ezra text (11th century AD)
(http://www.answering-christianity.com/76f0bdea508f2723.png)
Masoretic Text (10th century AD)
?? ???? ??????? ????? ???? ????? ???? ???? ???? ????? ????? ?????
You can see that is totally different... the Text of Ibn Ezra is much longer ! The text given by other Rabbis is also different !