Be VERY careful! That Marshall HAS to have a load impedence of 8 ohms or greater! Lower impedence will kill the thing in short order, no pun intended. As with most solid-state output amps, you will be forced to go with a higher total speaker impedence if you want to run external cabs with the internal speakers. If you have 8 ohm cabs, you will have to make a series adapter to put the internal speaker in SERIES with the external cab for a total impedence of 16 ohms. Because this is a stereo output amp, you will have to do this twice, once for left, once for right. A parallel "splitter" cord will give you too low an impedence and hurt your amp.