I have a Mackie Onyx 1620 and a Shure SM7B on channel 5. The board has a Low-Cut Switch on the channel. When it is engaged it causes DC Offset. Take a look:

http://www.pfiggianimsc.org/wave.html

Should this be expected? The graphed scale at the left represents the Amplitude ruler in my waveform Editor.

Thanks.

-paul.