I am doing the same thing splitting to dual signals with mic in channel b and guitar in channel a. The gain knob is kinda touchy and my mic is less than superior. Some suggest the vintage pre for vocals but it sounds muddy to me. The harmonizers sound cool. Im researching to find out how to have stereo effects for guitar and not for vocals but using dual paths. There isnt very much information about using a mic so hopefully others will come out and share.