Well then show me the evidence so I can present it to all those Biblical scholars who are hiding it from us.
Does those DSS have a full copy of the Book of Isaiah in Greek to compare to the Book of Isaiah in Hebrew discovered in the DSS??? How well does your Septuagint match that Isaiah manuscript????
I guess that vaunted Septuagint that was all over the Christian world just disappeared in Origen's day, right??? It was so fixed and common that he couldn't find it, but had to undertake an expensive venture in order to write one for a generation of Sola Septuaginters to come.
You might remember that the original LXX was kept in the Library at Alexandria, so it is hardly surprising that we don't have it: the Library was damaged by riots of Coptic monks, and destroyed by the Muslims.
The fact is the Masorete, in terms of oldest extant manuscript, is about 700 years more recent than the LXX. It is simply a scholarly error to regard it as more 'authentic' than the LXX simply because it is in the same language as the hypothetical ur-text.
If you want to focus on Isaiah, go ahead, and join the Christ-denying Jews in removing the prophecy of Christ's virgin birth from the Scriptures. Yes, on that point, Qumran agrees with the Masorete.
But you still have to explain away the Hebrew copies of Tobit and Ecclesiasticus found at Qumran, and the fact that Genesis, Eziekiel, Proverbs and several other books have the reading of the LXX confirmed against that given in the Masorete by the Dead Sea Scrolls.
And I still haven't heard a reason for preferring textual transmission via anti-Christian rabbis to textual transmisison by Christ's Church. Will one be forthcoming?