A bit of caution here, a speaker gives resistance to the amp and without that the amp will self destruct. What you need to do is one of several things
1. provide the aforementioned DI box but it must be of the speaker simulator type, this lets the amp function as normal and you take a signal from the simulator and record that. So your signal goes- amp- simulator- speaker.
2. What I have is kettle element that came a 32 amps. A friend cut it in half and rewound it to 16 amps and I plug that in the speaker out. I take a sugnal from the first stage out and put that thru a DI box and record that, the advantage of this is it is silent and you can do it in the middle of the night without someone calling the cops. Amp- resistance box. Thats it.
good luck