Not quite as simple as that, @peterhuppertz and @PamPurrs . . . 
A Rumble 100 is a “combo” amp, meaning the head is built-in and connected to a speaker, in this case a 1 x 12".
The Rumble 800 is a separate amp “head” and needs to be matched with an appropriately sized speaker cabinet, of which there are several. It gets really involved. Really quickly.
Here’s an old thread about this: 4/8 Ohm Impedance Explanation
Cheers, Joe