An emitter-follower has no voltage gain, no amplification of voltage levels. But it has current gain because its input current is low when its output current is fairly high.
We do not know if your amplifier and crossover network are able to drive many LEDs so the transistors are used to reduce the current required by the LEDs.
Your 2N5551 NPN transistors have a high voltage rating but a low current gain. Their current gain is poor when the current is only 70mA or more.
But a 2N4401 NPN transistor has high current gain when the current is 400mA or less.
A 2N3906 PNP transistor has a low current gain when its current is 50mA or more and its max allowed current is only 200mA.
But a 2N4403 PNP transistor has high current gain when the current is 300mA or less.
We do not know if your amplifier and crossover network are able to drive many LEDs so the transistors are used to reduce the current required by the LEDs.
Your 2N5551 NPN transistors have a high voltage rating but a low current gain. Their current gain is poor when the current is only 70mA or more.
But a 2N4401 NPN transistor has high current gain when the current is 400mA or less.
A 2N3906 PNP transistor has a low current gain when its current is 50mA or more and its max allowed current is only 200mA.
But a 2N4403 PNP transistor has high current gain when the current is 300mA or less.