if you remove the batteries for a full 48 hrs and just let it sit, maybe the unit will forget the calibration and come back in default setting?
Can you please try it?
Sometimes these things have and save to a volatile flash memory that gets wiped when there is no power, or if a capacitor is inside to help the settings survive a battery change, you have to wait for the capacitor to drain of all stored power, and then the settings are lost, which is what we would WANT in this case.
and then after waiting 48 hrs with no battery and no USB connections during that time, try again, without doing the calibration, and see if it sounds like the original, BEFORE calibration...
Please try this and see.
This ^ is what I'd do first, and then I would open it up and look for a shunt or dip switches or a reset jumper, but I may have more experience investigating unknown electronics than most folks here and I AM NOT RECOMMENDING anyone else try this, and I am not responsible if you do open it and break it - proceed at your own risk and only if you are knowledgeable with electronics, like using a logic probe, oscilloscope and soldering iron, if not, then I recommend DO NOT OPEN IT!
Mahalo!