Here's what Luther had to say about firm Amens, courtesy of Scott Diekmann at Stand Firm.
Quoting from Luther's "A Simple Way to Pray (...for Master Peter the Barber)":
Finally, mark this, that you must always speak the Amen firmly. Never doubt that God in his mercy will surely hear you and say "yes" to your prayers. Never think that you are kneeling or standing alone, rather think that the whole of Christendom, all devout Christians, are standing there beside you and you are standing among them in a common, united petition which God cannot disdain. Do not leave your prayer without having said or thought, "Very well, God has heard my prayer; this I know as a certainty and a truth." That is what Amen means.