Which prophesied - Sung hymns and prayed. But the Targum understands this of prophesying in the proper sense of the term; and therefore says, "Who prophesied by the Holy Spirit." Jarchi is of the same opinion and quotes the case of Elisha, 2 Kings 3:15; While the minstrel played, the hand of the Lord [i.e., the spirit of prophecy] was upon him.
Under the hands of Asaph - That is to say, “under the direction of Asaph” - who himself “prophesied,” or performed the sacred services, “under the direction of the king.”