If the power amplifier is powered with +12V AND -12V then the total supply voltage is 24V. An amplifier is not perfect, it has losses so its maximum output is not 24V peak-to-peak, instead it is about 20V peak-to-peak.
20V peak-to-peak is 7.1V RMS which produces only 6.3W into an 8 ohms speaker or maybe 10W into a 4 ohms speaker.
100W into 8 ohms is a signal that is 28.3V RMS which is 80V peak-to-peak. Its amplifier would need a power supply that is about +44V AND -44V.
Your "60W" amp that is powered by 12V externally might be a car amplifier with 4 amplifiers inside (left front, right front, left rear and right rear) so each amplifier produces 15W "peak" into a 4 ohms speaker or about 9W "peak" into an 8 ohms speaker. 15W "peak" is 7.5W RMS and 9W "peak" is 4.5W RMS. Each of its amplifier has two amplifiers arranged in a bridge so that the 12V supply is effectively doubled to 24V. Peak power is fake power, RMS power is real true power.
Most power amplifiers DO NOT need a voltage regulator. STK amplifier modules were made by Sanyo but recently Panasonic took over Sanyo and the amplifier modules are not made anymore.
Notice on the schematic the power supply pins +VCC and -VCC are marked "!" which means DANGEROUS VOLTAGES. If you do not know how to measure the voltages in your amplifier with a multimeter then get an electronics geek to do it for you.