Reading up some more, it appears that the Supersix Hi-Mod is the only model of Supersix using the BB30a, so you just have a BB30. So the Wheels MFG BB30 to Shimano will work. There are others that are direct conversions, like Kogel, but only if you feel like shelling out $175 for press-fit. The Wheels BB will run about half that, and threaded will be the best bet to prevent future creaking.