The AudioBox iTwo has an "input Source Button" (the one with the guitar icon) to increase the gain of the preamp. This seems however only to works for unbalanced sources like e.g. electric guitar or bass. However, microphones use to have balanced connections and in this case pressing the "input Source Button" has no effect at all. The amplification is correspondingly not sufficient for typical dynamic microphones. I didn't check with the Shure SM7B but with the Shure SM58 the signal is too week even in maximum position of the input gain buttom.
The support could not explain, why the "input Source Button" doesn't work with symmetric sources. So this piont is still unclear to me.