That is simply happening because you are overloading the output amplifier by mismatching its output, if there wasn't any protection circuitry on the amp it would blow through overheating etc in a very short time. Do NOT put any speakers in parallel with the existing ones, and check that the speakers you are connecting in place of the original one has EXACTLY the same impedance rating (resistance) as the original. (i.e: if the original is 8ohms replacement MUST also be 8ohms, do NOT use a 4ohms one, although you could use 16ohms, but just wouldn't get the same power output)