Whats the correct impedance setting for an amp with an extension cabinet connected?
As with all tube amps the correct impedance setting for the amp is the one that is the same as the total impedance of all the speakers that are connected to it. Set the impedance selector in the back of your amp to match the load (speaker) impedance that you are using. You will get the same performance (but not necessarily the same tone) with a 16 ohm cabinet by setting the selector to 16 ohms as you will if you use a 4 ohm cabinet and set the selector to 4 ohms. If the amp has no impedance selector (Fender Twin etc.) then choose a combination of speakers that comes close to the rated impedance… preferably lower. The speaker output jacks on a Marshall JCM 900 are connected together in parallel so the total impedance of the internal speakers and the extension cab together will be a parallel combination. See the following questions to determine what the total load impedance will be. A 100 watt Marshall will produce up to 175 watts of output power when running full blast. You will event
