I had the opportunity to test a Focusrite Scarlett 2i2 and though it also produces low levels with both the Shure SM58 and the Audio-Technica ATR2100x, the levels are higher than with the Studio 24c.
A friend tested his Neat King Bee mic on both these audio interfaces and was getting good levels. So the takeaway is that Studio 24c really isn't a good choice for paring with anything but a high end mic. Which defeats the purpose of having a budget interface.
Even though the Scarlett is a bit more expensive and lacks the two MIDI ports of the Studio 24c, it's still the better choice because of its stronger pre-amp and it is considerably smaller.
