What's your head unit? Head units may produce max rated voltage at or BELOW max volume. Head units can also cause distortion. Even a name brand head unit can cause distortion at or below max volume.
If you're using the bass knob on the head unit, you are also increasing the voltage at that lower frequency. Setting the bass knob to flat, and eq to flat is going to be best your chance at not having distortion. Your head unit is more likely to be causing the clipping then your amp.
If your amp is set at rated power, (and rated power has a low THD, then it's not causing the clipping). All this would do is sound the same as a an amp pushing power and having the volume turned down.
If you have an O-scope, then set all your eq, bass anything to flat, and recheck your signal with lower than max volume on your head unit. Turn it down one, then turn up the gain. When your signal starts to distort, you can check AC voltage like this and see what wattage you are getting. You didn't say the amp. Your amp might be underrated. You (very slim chance) MIGHT be able to get away with a different head unit with higher preout voltage. Spending the time to do this, will at least help you find where the max volume you SHOULD use on your head unit.