the input sensitivity allows you to match the output voltage of the head unit with the input voltage range on the amplifier. one quick and safe setting is to set the input sensitivity to the RCA output voltage of the head unit. on most amps, the input sensitivity has labels in Volts. if not, it's a cheap amp and you'll have to refer to the manual for the input voltage range, then adjust the dial as needed assuming the gain is linear.
so if you have a 2V output voltage on the head unit, set the amp sensitivity to 2V (or lower). this is the easiest and safest method, but you won't get full power from the amp without maxing out the head unit and playing 0dB test tones. not a bad thing, but worthy of noting.
you just want more speaker volume and less sub volume.
as for bass boost on the amp, that can be a good thing if you turn down the Bass EQ on the head unit. this combination will protect the speakers by removing some bass, then restoring it at the sub.
in addition, if you set your gain based on output voltage, then a little boost is just fine. even some loudness is fine since you're gain is lower than it could be.