Can I connect the Torpedo Live to an external cabinet and play at low volume?

This would be the role of an attenuator, such as the Torpedo Reload

The Torpedo Live will give you a line output but no control over the cabinet that you would plug into the THRU output (check the manual for more information).

If you do need this function together with Two Notes's speaker-cabinet simulation, a Torpedo Reload + Torpedo C.A.B. combo will bring you the best of both worlds.