Question:Is it true that Amos pronounced a curse upon those who, like David, introduced instrumental music into Hebrew worship?
David did not introduce instrumental music into Israelite worship, God did. Long before David lived God commanded the Israelites to blow trumpets as part of their worship (Leviticus 23:24; Numbers 10:10). Prophets used instrumental music, "After that you shall come to the hill of God where the Philistine garrison is. And it will happen, when you have come there to the city, that you will meet a group of prophets coming down from the high place with a stringed instrument, a tambourine, a flute, and a harp before them; and they will be prophesying" (I Samuel 10:5).
"Woe to you who are at ease in Zion, and trust in Mount Samaria, notable persons in the chief nation, to whom the house of Israel comes! Go over to Calneh and see; and from there go to Hamath the great; then go down to Gath of the Philistines. Are you better than these kingdoms? Or is their territory greater than your territory? Woe to you who put far off the day of doom, who cause the seat of violence to come near; who lie on beds of ivory, stretch out on your couches, eat lambs from the flock and calves from the midst of the stall; who sing idly to the sound of stringed instruments, and invent for yourselves musical instruments like David; who drink wine from bowls, and anoint yourselves with the best ointments, but are not grieved for the affliction of Joseph" (Amos 6:1-6).
You missed the point of Amos 6. God is describing people who live in idle luxury. They are so wrapped up in their own pleasures that they don't see the doom coming at them on the horizon. Isaiah made a similar charge, "Woe to those who rise early in the morning, that they may follow intoxicating drink; who continue until night, till wine inflames them! The harp and the strings, the tambourine and flute, and wine are in their feasts; but they do not regard the work of the LORD, nor consider the operation of His hands. Therefore my people have gone into captivity, because they have no knowledge; their honorable men are famished, and their multitude dried up with thirst" (Isaiah 5:11-13). We are not talking about people worshiping God, which is how you applied it. We are talking about people who spend their time on personal entertainment. These people thought themselves to be an equal of David's musical skills, but where David used his gifts to glorify God, these people were composing songs for themselves. God destroyed them for their sins: their laziness, their lack of care for their fellow men, and for their negotiations with other countries. God didn't destroy them because they used instrumental music in their worship of God.
God was worshiped with instrumental music under the Old Law because God required it of the Israelites.