I have the combo, which has a 16ohm 1x12 built in. So I was thinking a true "extension" cab should be either another 1x12 (@ 16 ohms), to ensure the load seem by the amp is "exactly" 8 ohms (yeah, I know inductance loads are variable over freq). Using a 2x12 (typical PRS config of two 16 ohm speakers in parallel) with the internal speaker (which would also be in parallel, electrically) results in an electrical load equivalent of 5.3 ohms, too low for the 8 ohm jacks, a bit too high for the 4 ohm jacks. Yeah, I could unplug the internal speaker and just use the 2x12, but I could have just got the HXDA head then - I like to use all available speakers. And 2x12 cabs in gold paisley are also just as rare these days, so perhaps a moot point.