I opened the OT again and re-soldered the joints in the right position. No change. BUT I noticed something very interesting.
I have the theory now, that every OT (modded and un-modded) makes a small phase change, and that over time. It’s not a simple phase inversion.
So I suggest to do the same test as me. Send an audio file to the OT digitally, play it back and record it again. Look at the waveforms closely or even align them perfectly and play them together and hear the phase cancellation.
It would be very interesting to read your opinions.