Since I don't know what a "solid" midrange amounts to, I can't answer the first portion of your question. As to why the soundstage shifts position (is that correct?), the answer would be the phase shift introduced by the tone controls.
I can only guess, but I would say the tone controls are not actually flat when centered. Can you duplicate the sound of the midrange by adjusting the tone controls? Don't concern yourself with the soundstage, just try to adjust the controls to tonally match what you are hearing with the tone controls bypassed.