With a total of only 70V, the output of the amplifier is about 64V peak-to-peak (or less) at clipping into 8 ohms if the amplifier uses normal transistors. Its output voltage and power are much less if the amplifier uses Mosfets.
64V p-p is 22.6V RMS. Then the power at clipping into 8 ohms is only 64W, not 250W.
There are millions of 64W amplifier circuits on the internet. The site where you found the sub-woofer control circuit has plenty of amplifier circuits.