I have an old amplifier that uses pairs of complementary transistors in the output (BD135 / BD136). If I change these transistors by others more powerful as TIP31 / 32, do get more power from the amplifier?
The output power of an amplifier does not depend only on the transistors used in the output. The output transistors are chosen according to a project which determines what is the current and voltage with which they must work. So, if you replace the transistors nothing changes in the circuit, as the new transistors will continue working with the same voltage and current for which the circuit is designed. The output power will remain the same and you still risk having problems if the characteristics of these new transistors are not in agreeable to what the project requires. More power means changing the whole circuit with a new calculation of driving steps, the supply voltage and everything else.