Torpedo Captor X and 16 ohms cabinet
When you use the Captor X, here are the different impedance behaviour in regards to the use of the attenuator:
1. VOLUME LEVEL on FULL position: the attenuator is bypassed. The output acts as a THROUGH output. The impedance shown to your amp is the cabinet's impedance. you are advised to use the corresponding output of your amp, in this case. (16 ohms in the case of a 16 ohms cabinet.)
2. VOLUME LEVEL on LOW or halfway position: the attenuator is in use. The impedance shown to the amp is the Captor X's one, which is 8 ohms. Use the 8 ohms output of your amp.
You can keep the cabinet connected at all times to the Captor X.
If your cabinet has an impedance of 4 or 16 ohms, you will be in case N°1. Using the 8 ohms output of the amp, you have an impedance mismatch but a tolerable one. This article will help you understand more about impedance mismatch and what you need to be careful about: