On the rear, you'll find a single speaker out and inputs for 4,8 and 16 Ohms from the amp. Say you have an amp that offers a 16 Ohm output only ( Current Vox AC4 models, certain small Marshalls ), but you want to drive a 4 Ohm load, plug the amp into the 16 Ohm input and whatever cab you want into the output. Even with the output level on full, there's some attenuation ( around 3dB ), but better that than risk your output transformer. Piccy from Thomann Note the inputs at the base. Match that to the amp. That's all there is to it.