Can you run an amp without a sub?

You probably could run your subwoofer off of the rear speaker outputs of your head unit, but you probably would not like the result.

It takes a great deal of energy to make bass tones and the power output of your radio is relatively limited.

The 'subwoofer volume' option is so that you can set the line-level output to a separate subwoofer amplifier and keep the subwoofer level proportionate to the main sound output.

Good luck.