I have an old amplifier that uses pairs of complementary transistors in the output (BD135 / BD136). If I replace these transistors with more powerful ones like the TIP31 / 32, do I get more power from the amplifier?
The output power of an amplifier does not only depend on the transistors used in the output stage. The output transistors are chosen based on a design that determines the current and voltage with which they must work. So if you change the transistors you will have no change in the circuit because new transistors will continue to work with the same voltage and current for which the circuit was designed. The output power will remain the same and you still run the risk of having problems if the characteristics of these new transistors are not in accordance with what the project requires. More power means changing the whole circuit, with a new calculation of the excitation steps, the supply voltage and everything.