If the output of the preamp is clipping then either reduce its input level or reduce its gain.
If the preamp is not clipping but the power amp is, then turn down the volume control because that is what it is for.
The output of the power amp when it has a 9V supply and a 16 ohm load is 250mW at clipping. That is enough to blow up headphones and destroy your hearing.
If both 16 ohm ear transducers are connected in parallel then the total power is 800mW at clipping.
If the preamp is not clipping but the power amp is, then turn down the volume control because that is what it is for.
The output of the power amp when it has a 9V supply and a 16 ohm load is 250mW at clipping. That is enough to blow up headphones and destroy your hearing.
If both 16 ohm ear transducers are connected in parallel then the total power is 800mW at clipping.