The Septuagint v. the Masoretic text

There is a lot of debate about the most accurate bible text these days so I am going to compare the text in the new testament where it quotes the old testament and compare these quotes to the Septuagint and the masoretic text. The masorectic is the Hebrew text that most bibles use for their old testament translation. The septuagint is the greek text that was translated from the Hebrew text in the second century BCE. The hebrew text that the Septuagint was taken from is not the masorectic hebrew text because the masorectic text did not come into existance until about the ninth century CE.

Here is the first N.T. quote. I will be using the KJV for the new testament and the masoretic comparison.

Hebrews 10:5,6
" Wherefore when he cometh into the world, he saith, sacrifice and offering thou wouldest not, but a body hast thou prepared me.
In burnt offerings, and sacrifices for sin thou hast no pleasure."

The Masoretic (MT)reference from which Hebrews 10:5,6 quotes
Psalm 40:6
"Sacrifice and offering thou didst not desire, mine ears hast thou opened: burnt offering and sin offering hast thou not required."

Now compare the Septuagint (LXX) reference from which Hebrews 10:5,6 quotes.
"Sacrifice and offerings Thou didst not desire, but preparest for me a body; whole burnt offerings and offerings for sin Thou didst not require.

See how the N.T. matches the Septuagint and not the masoretic text here. Here is another one.

Matthew 12:21 "And in his name shall the Gentiles trust."

Isaiah 42:4 M.T. The last part.
"and the Iles shall wait for his law."

Now the Septuagint
"In his name shall the gentiles trust"

Matthew 15:9
"But in vain they do worship me, teaching for doctrines, the commandments of men"

Isaiah 29: 13 the last part.
"..but have removed their heart far from me, and their towards me is taught by the precept of man."

The Septuagint
"In vain do they worship me, teaching the commands and the doctrines of man."

Here are some other comparisons to look at to show how the N.T. quotes match the Septuagint.
Mathew 21 :16 the last part: Psalm 8:2 the first part.
Matthew 3:3: Isaiah 40:3
Romans 2:24: Isaiah 52:5 the last part.
I Peter 4:18: Proverbs 11:31

There are a lot of other quotes in the N.T. the match the Septuagint over the Masoretic text if anyone is interested in seeing more of them then let me know in the comment section.
One point I would like to make is that even though the N.T. quotes match the Septuagint I do not nessesarily believe that the N.T. writers were actually quoting the Septuagint they could have been quoting a Hebrew text that simply matched the Septuagint. The dead sea scroll text is good evidence of this.