What duz level adjustment do
The level control controls the input level coming from your head unit (receiver) to the amp.
Your Sony users manual is a little vague on how to best adjust the level and other controls.
Here is one method that some installers use and works well with most amps.
Most 10" subs sound best between about 80-100hz and below, so start out by setting the LPF at about 80hz. The HPF will not be used. Next turn the bass boost and gain all the way down. Turn on the radio and set all tone controls, bass, midrange, treble to flat, usually "0" on most head units. Turn the volume up to approximately 3/4 volume level or just until you begin to hear distortion. Now, back the volume down until the distortion is gone. Next turn up the gain control on the amp until you hear the subs start to distort then back the gain down until the distortion disappears. Next turn the bass boost up again until the subs begin to distort, then either back the bass boost down or back the gain down until the distortion is gone. You may need to play around with the bass boost and gain controls to get exactly the sound you prefer.
Hope this helps.