Claims are made that the DSS manuscripts often agree with the Septuagint over the Masoretic text. This is highly misleading and mostly based on nothing more then that the DSS contains some of the Apocryphal books that were included in the Septuagint. Which includes stuff written to be added to Daniel and Esther. The Isaiah Scroll certainly matches the Masoretic text far more then the Septuagint.
I defend Almah meaning Virgin without appealing to the Septuagint at all. In fact using the Septuagint to prove that is undermined by that the Septuagint also used Parthenos of Dinah after she is raped in Genesis 34. The Hebrew uses neither Almah or Bethulah in that chapter.
I'm going to quote from a Jewish website incorrectly asserting that Almah doesn't mean Virgin because of some important facts it points out about the Septuagint.
I btw believe Matthew was originally in Hebrew so certainly not quoting The Septuagint.The original Septuagint, translated some 2,200 years ago by 72 Jewish scholars, was a Greek translation of the Five Books of Moses alone, and is no longer in our hands. It therefore did not contain the Books of the Prophets or Writings of the Hebrew Bible such as Isaiah, from which you asserted Matthew quoted. The Septuagint as we have it today, which includes the Prophets and Writings as well, is a product of the Church, not the Jewish people. In fact, the Septuagint remains the official Old Testament of the Greek Orthodox Church, and the manuscripts that consist of our Septuagint today date to the third century C.E. The fact that additional books known as the Apocrypha, which are uniquely sacred to the Roman Catholic and Orthodox Church, are found in the Septuagint should raise a red flag to those inquiring into the Jewishness of the Septuagint.Christians such as Origin and Lucian (third and fourth century C.E.) edited and shaped the Septuagint that missionaries use to advance their untenable arguments against Judaism. In essence, the present Septuagint is largely a post-second century Christian translation of the Bible, used zealously by the Church throughout its history as an indispensable apologetic instrument to defend and sustain Christological alterations of the Jewish Scriptures.For example, in his preface to the Book of Chronicles, the Church father Jerome, who was the primary translator of the Vulgate, concedes that in his day there were at least three variant Greek translations of the Bible: the edition of the third century Christian theologian Origen, as well as the Egyptian recension of Hesychius and the Syrian recension of Lucian.1 In essence, there were numerous Greek renditions of the Jewish Scriptures which were revised and edited by Christian hands. All Septuagints in our hands are derived from the revisions of Hesychius, as well as the Christian theologians Origen and LucianAccordingly, the Jewish people never use the Septuagint in their worship or religious studies because it is recognized as a corrupt text.The ancient Letter of Aristeas, which is the earliest attestation to the existence of the Septuagint, confirms that the original Septuagint translated by rabbis more than 22 centuries ago was of the Pentateuch alone, and not the Books of the Prophets such as Isaiah. The Talmud also states this explicitly in Tractate Megillah (9a), and Josephus as well affirms that the Septuagint was a translation only of the Law of Moses in his preface to Antiquities of the Jews.2Therefore, St. Jerome, a Church father and Bible translator who could hardly be construed as friendly to Judaism, affirms Josephus’ statement regarding the authorship of theSeptuagint in his preface to The Book of Hebrew Questions.3 Likewise, the Anchor Bible Dictionary reports precisely this point in the opening sentence of its article on the Septuagint which states, “The word ‘Septuagint,’ (from Lat. septuaginta = 70; hence the abbreviation LXX) derives from a story that 72 elders translated the Pentateuch into Greek; the term therefore applied originally only to those five books.”4In fact, Dr. F.F. Bruce, a preeminent professor of Biblical exegesis, keenly points out that, strictly speaking, the Septuagint deals only with the Pentateuch and not the whole Old Testament. Bruce writes,The Jews might have gone on at a later time to authorize a standard text of the rest of the Septuagint, but . . . lost interest in the Septuagint altogether. With but few exceptions, every manuscript of the Septuagint which has come down to our day was copied and preserved in Christian, not Jewish, circles.5
There are also quotes of the Septuagint version of the Torah in the Talmud that do not match the standard Septuagint texts we have today.
At any-rate it is wrong to claim that the Masoretic Hebrew text was written by the Rabbis, in fact they are Kariate texts, Kariate websites love to point out that the Hebrew Texts the Rabbis use today are based on Kariate preserved manuscripts. Kariates can at times be just as hostile to Christianity as the Rabbis, but they are the Sola Sciprtura believers of the Jewish world, they revere The Word and would have preserved it accurately. When it comes to respect for Scripture I certainly trust them more then the Eastern Orthodox Churches, or the Alexandrian Early Church Fathers.
The Holy Spirit sometimes works though unbelieving individuals, like Balaam. So I have no trouble believing He used the Kariates to preserve the proper Hebrew TNAK.
There is also a somewhat older then the standard text of the Septuagint that does not have Kenen added to the Genesis 11 genealogy like our standard texts do. This shows me that Kenen was added by Christian copyists who thought it's absence from Genesis 11 was a problem for Luke 3. But I believe the issue is that Luke's is a genealogy that is willing to sometimes go through Women, but always naming the male of that generation. I think Kenen was an at least 15 years older brother of Selah and that Selah married Kenen's daughter.
Likewise Josephus and many Early Church Fathers who seem to be using the Septuagint as their source for this genealogy don't include the added Kenen either.
The reason the New Testament often seems to match the Septuagint is because Christian copyists conformed the Septuagint to match the New Testament refrences. As for why they may seem to not match the Hebrew, I think many NT quotations of the OT were not meant to be exact word for word quotes. If Deuteronomy can express the Ten Commandments differently then Exodus then I certainly have no problem with Jesus expressing things a bit differently during his ministry. Jesus own quotes were expressed differently at different times he said them (and thus recorded differently in different Gospels).
And these people making a big deal out of where the Septuagint seems to match the NT ignore places where it doesn't. Like the spelling used for Solomon, Solomon is the spelling Luke used, no NT spelling puts an Alpha in there. But the Septuagint spells is Salomon. Also the Septuagint never uses the same word for Comforter that John's Gospel and Epistles used.
The Septuagint "translates" Tarshish as Carthage, which is clearly wrong, Carthage didn't exist yet in Solomon's day. And was a Canaanite/Sidonian colony in Phut's territory, not Japhethite.
The Septuagint adds references to Gog that sadly some people today want to build their Gog doctrines on, but really don't make sense. One replaces Agag in Balaam's oracles. Balaam talked a lot about Amalek in those oracles but never Magog. The other is in Amos in a context that people use to identify Gog with Apollyon.
Also I couldn't even find in the Septuagint the Elam Prophecy of Jeremiah 49.
Also the Septuagint numbers for Genesis 5 fail to match the meaning of Methuselah's name (his Death shall Bring)) by having him die the same year as The Flood like other version do.
The standard texts we have of the Septuagint are in the exact same Manuscripts as the Alexandrian Bibles used by modern Scholars to justify all kinds of mutilations to the New Testament.