Re: Extension cab. More volume? Better spread? Stereo?
One question at a time people! Lol.
Stereo: No, in order to achieve true stereo you have to have more than one poweramp driving more than one speaker set. It's the difference in signal (sometimes only very subtle) that gives something a true stereo effect. You can however simulate stereo by moving speakers around and using delay and reverb.
Impedance: yeah, your summary isn't exactly re technical explanation but it's near enough right. Reduce the impedance, you get more from the amp.
Full stacks: this is slighty different. With solid state amps, the volume per speaker isn't louder necessarily, itdepends on what cabs you're ysig, you just get more sound.
Vale amps however put out the same amount of power regardless of how many speakers are connected. So take your dual rec, slap a couple of four twelves under it and valve amps put out I'm such a way that having two cabs chuck out the same kind of power that one would on it's own does make it seem quite significantly louder. The individual speakers arent louder, but justa ssinf one speaker to a valve amp makes the 'perceived intensity' notably higher (cabinetry assumed to be the same). It's to do with transient responses and spl which I'm not in the mood to attempt to explain in a way anyone can understand. I welcome somebody smarter than me to have a go lol.