Rev 9:15 directly follows 9:14, describing the release of the four angels as commanded, resulting in the killing of a third of mankind.

Rev 8:2 introduces the seven trumpet angels, including the sixth angel who is commanded in 9:14 to release the four bound angels.

Rev 8:6 shows the seven angels preparing to blow their trumpets, setting the stage for the sixth angel’s action in 9:14.

Rev 16:12 also involves the Euphrates river in a judgment (the sixth bowl), echoing the river’s role in the sixth trumpet of 9:14.