I think both are effects of loading, and under different circumstances. I may still not have a clear grasp, since your description isn't fully spelled out.
Your output is feeding a mic pre? I'm assuming that from what I'm reading. And assuming lo-Z input position.
In bypas, we are switching an entirely passive signal through transformers, or not.
In bypass, with lo-Z input and hi-Z output, you are stepping up impedance with the input transformer and then trying to dump it into a lo-Z input again (your preamp). So you probably have mismatch loading losses, as your mic is now seeing a very low load impedance reflected from the preamp. Conversely, the preamp is being driven by an impedance that has been stepped up out of the expected range.
Switch to bypass with lo-Z input and lo-Z output, the transformers are then doing reciprocal step up/down and you have correct match for the preamp again, and only transformer insertion loss rather than mismatch loading loss. Probably an actual gain change of -1-3 db over plugging straight into your pre.
Unit active, all sounds normal. hi-Z on output is probably recommended load, and not actual, and can still drive your preamp. lo-Z puts transformer in path, which steps down level.
