I have a Peavey 158 guitar amp which has an output jack called preamp out. Is this the same as a line out? Is it safe to plug into a sound card without a DI box?
A preamp out is simply a line-level output from your preamp (makes sense, right? . It's safe to plug into a sound card's line in, all right, but it's not the same as the "final" output of your guitar amp the way a "line out" jack on a guitar amp would be.
The usual reason people use these is to plug their amp into a bigger power amp without blowing anything up, but you're generally better off micing it into the PA for that kind of stuff.