You have two separate power amps built into you head from what I can see in the manual layout. (See here:
http://s3.amazonaws.com/samsontech/r...000_manual.pdf)
The top set of two outputs and bottom set of two outputs represent the two different power amps. Each set will put out 250w @ 4 ohms, 170w @ 8 ohms, and 100w @ 16 ohms total impedance. For example, if you plugged four 8 ohm cabinets in the amp, it would be putting a 125w into each cabinet to a total tune of it's full 500 watts of power.
The power each cabinet receives individually varies based upon it's own individual impedance, the impedances of all other cabinets connected to that specific power amp, and the capabilities of the power amp.