If you are connecting two 8 ohm cabs to your amp you need to plug them into the 4 ohm speaker output.
Read the owner's manual. It's really easy to blow sh%R up in a tube amp if you don't know a few basic rules.
I just read the manual. Thank you for pointing to it as well.
"Two 4 Ohm, Two 8 Ohm and one 16 Ohm jack are provided for speaker interfacing. Both the Dual and
Triple Rectifier Heads are not very sensitive to speaker mismatches and will not be damaged by them"
Seeing as it says this....I guess there's nothing to worry about. thanks!