BE WARY: I may be wrong, but it is my understanding that simple plugging cabinets into one another will not lead an ohm value representing series wiring unless your jacks have a series/paralell switch. I think that even though you can plug the cabs into one another the speakers are still wired in paralell... Worst case throw a multimeter on there before you plug it into the head and make sure your not going to smoke anything.... At least not in the good way.
This is correct, just plugging the cabs into each other will create a parallel load, effectively reducing impedance.
I'm not sure if this is an issue with tube heads, though.