So I know the basics of impedance matching, but I've got some questions. I've got a Dual Rectifier and a JCM800 with a 1960A cab.

The cab has 2 inputs, one for 4 ohms and one for 16, and then a stereo/mono switch. I had always thought that the stereo switch was for using it as a fullstack, so you would set the amp to 16 ohms, run a cable to the cab set to stereo, then run another cable out of the second jack on the cab into a second cab. But from looking through Marshall's faq, it seems like the stereo setting is for only using 2 of the 4 speakers in the cabinet. Does this mean I can run both of my amps, each at 8 ohms, into each of the inputs, and basically use it as two 2x12s in one cabinet? Or am I stupid and is this a bad idea. It sounds right, but I don't want to try it and fry one of my amps.

I'm also building a 2x12 for my room (I keep the 4x12 in my basement for when my band practises). The jackplate has two inputs, so I'm thinking of wiring it for 8 ohms in one and 16 in the other. I'm also toying with the idea of running both cabs as 3/4 stack just for the hell of it. Now I'm confused about how to do this. Say I have two 8 ohm cabs. The manual says to run two cables from the 4 ohm outputs to the cabs for a correct match. Wouldn't that total the output of the amp at 8 ohms, and the impedance of the cabs at 16 for a safe mismatch? Or am I misunderstanding something? The Marshall has me even more confused, because it has two outputs, but an impedance selector switch. So if ran it from each input into two 8 ohm cabs, would I want that switch at 16 ohms, since the cabs are 8 + 8, or would I set it to 8 ohms, since I'm using 8 ohm cabs?