Maybe this is well-known here already, but if you're using input transformers, you need to be sure that your signal cables go from their outputs directly into the line inputs that you want to drive, and are as short as possible--less than three feet long for sure.
This is to avoid high frequency losses due to the capacitance of the cables. Step-up transformers don't just step up the voltage; they also step up the source impedance of your microphones by the square of the turns ratio.
--best regards