You would be much better off micing the cab IMO. If you use the slave out, like soundpurist said, you'll need some sort of load on the output transformer, otherwise you'll cook it, and possibly the power tubes. That's not a cheap repair either.
Then, you'll need some sort of cabinet emulator. The slave out is a line-level signal taken from the preamp. You get no sort of power amp tone, or speaker tone with it. It sounds buzzy, and thin. Maybe good for cleans, but not good for distortion.
IMO, it's more hassle than it's worth. If you're trying to do late night recording, you'd be much better off waiting until the day and crank it up.