what really changes is the output impedance being able to use loads of 4 ohms or less depending on the transistors that you place in parallel and when using loudspeakers of lower impedance logically increases the power, but you should take into account that the consumption on the source also increases, to place transistors in parallel only join them with the originals, but place the PTC resistor that brings the circuit to each transistor, e.g. If the PTC is going to issue, the new ones must also have their PTC, independent of the original ones, I advise you to be all the transistors of the same brand and heading so that they work as twinned as possible, greetings and luck.