stereo or mono

I'm confused as to the impedance of my Pro Tube IX...Apparently it's a stereo amp, 450W @ 4 ohms each side in stereo, but does that mean I can have two 4 ohm cabs??? Or do I want two 8 ohm cabs for a total impedance of 4 ohms?? I also have the option of running it in mono too. I'm new to...