You need a preamp for a guitar, but not for a line-out on a guitar amp. A line out is strong of a signal to run into a preamp. Line level signals are waaay stronger than mic signals - That's what a preamp does, it boosts mic level to line level. A line-out is already boosted to line level (which is why it's called a line out). Run those line-out's directly to a line-in on your interface. No preamp required.
Which is actually the opposite of what you asked now that I re-read your post, so let me switch it around: You can't plug a mic into a line-in - the line-in is expecting a strong line level signal. A mic signal is very weak, and a line-in will NOT boost it at all. You can eat the mic and scream into it, and you'll be registering peaks of like -60dbFS, which is barely audible.
