I think the only remaining question is the last one I posted:
"... I suspect you may have switched the tone and balance potentiometers - that would explain the difference. We can check that further by just measuring the input resistance of the Stereomour with the tone control centered and at each extreme. I think this is worth pursuing, just to be sure the amp is working correctly. "
You can access the inputs most easily from the cable connecting the Magni to the Stereomour. Just pull the RCA hacks from the Magni, leaving them connected to the Stereomour. With the balance control centered, measure the resistance between the center pin and the outer shield of the jack, on both channels. Repeat for the balance control all the way clockwise, and again with it all the way counter-clockwise.