OK, more info, now I have had a chance to check out the back of the amp. There is a 16 ohm and an 8 ohm output. The 16 ohm output is actually labelled 16 (4), which is where I was getting confused with what options were available. This is the output that I have used from day one, and have had the cabinet in mono mode.

If I switch to 8 ohm, for instance if I am using an 8 ohm heil talk box, I would switch to 8 ohm, does this still get split amongst the speakers, so each speaker handles 2 ohms?