Where does the cab simulation sit in the signal chain?

There are actually two separate cab-sim systems at work here. The first is a purely electronic cab sim, sitting just before the Line-Out / Headphone output. It's set by the Voice switch to emulate what the speaker would sound like (full-range or guitar-mode), and it's completely independent of the speaker, so you can unplug the speaker or turn the Master all the way down and still get it. The second is a more "organic" system feeding the Direct Out on the back, which actually uses your speaker's real response as part of the tone shaping. Think of it as a mic replacement, so your speaker needs to be running at a reasonably normal volume for it to sound its best.