wouldn't shorted be NO load, not a huge load? And open is basically infinite resistance, or HUGE load. If I understand it right, Higher loads than designated are more likely to blow transformers, while lower loads are more likely to blow power tubes. Mesa at least says it's ok to try mismatches, but I think they usually mean 1 step, 4 -> 8 or 8->16 or the otherway. Mesa are probably overbuilt enough to take a 2 step change, but I don't know for sure. If it's a multispeaker cab, might be easier to rewire it to lower the resistence if possible.