They are equivalent, but not identical.
In fact some old QSC amps went from the one to the other with no other changes.
But the problem is that the transistors do not work alone. There are rows of them inthe power stage, and they must share current well, and they must track thermally. If they don;t, then one of the transistors can "hog" the current, and burn up. Likewise, as teh whole amp warms up, you want all the power semis to react to the temperature changes similarly
SO you really don;t want to mix types. If there is a row of four D555s, you don;t want a lonesome 15024 in the row. All of one or all of the other.
Now if this is stereo, you might be able to swap between channels to get one side all of a type. It does not matter if the left channel and right channel use the same parts as each other.
I usually find the best prices on those MJ150xx series transistors from Allied.