does adding an extra cabinet make you any louder, even though you're still at the same power output?
Yes - most amps are designed to deliver their full power into a 4Ohm load. Most cabinets (and combo speakers) are 8Ohm. It is only when you add an extension cabinet that the amp has a 4Ohm load (2 x 8 in parallel) and thus able to deliver the full beans.