General Instruction of the Roman Missal, #79h:
Final doxology: By which the glorification of God is expressed and is confirmed and concluded by the people's acclamation, Amen.
Jeffrey Tucker at NLM:
"The Amen need not be "great" but rather just two notes."
To this day, you'll be surprised how many people still call the Amen that concludes the Eucharistic Prayer the "Great Amen". Even in Jubilate Deo, there is only ONE Amen (tone as pictured here). Yet, the Amen you hear at Mass is usually a threefold to sixfold Amen, depending on which Mass setting you're using. Some even have unnecessary additional words, like "forever and ever" and that "a-word" that is forbidden right now.
Now, let's go back to the GIRM citation at the top of this post. Notice they only mention one word that concludes the Eucharistic Prayer - Amen. It doesn't even mention repetition here. Just Amen.
Here's another really good take - this one comes from Gavin, a former blogger who now posts at the Musica Sacra message boards:
Nothing in the GIRM, rubrics, or tradition (that I know of) requires the congregation to sing "Amen" more than once at any point in the Mass. Yet today every Catholic pewsitter knows that the IMPORTANT part of the Mass ISN'T the words "This is My Body" but when you have four chords and sing "A-A-MEN, A-A-MEN, A-A-A-MENNNN" and then repeat it. I've even heard catechists say that THAT is the point where the bread becomes the Body. Oh, and the scores for these "Great Amens" always have FFF as the closing dynamic. This HORRIBLY imbalances the Mass!
So when your priest sings "Through him, with him, in him" to the simple tone, just respond on the same note he used as the reciting tone: "Amen." If he uses the solemn tone (with the slurs on some syllables), respond according to the pitch he ends on "A-me-" and then move up a whole tone "-en." It's all so simple, no one can object to it if it's done routinely, and it makes SUCH a difference in how the Mass is perceived by the congregation.
Through this vision from Gavin, now you'll see the climax quickly shift (in the eyes of the average Joe/Jane in the pew) from the so-called "Great Amen" to the consecration and elevations.
Great posts, Jeffrey and Gavin!
PS: And the people say: AMEN!