I didn't keep a copy of those emails. But a few that I do remember is the non existing power supply decoupling inside the unit. If you use the TC-750 with a 12V battery there is no RF interference that I could hear. The other thing is with the dynamic range that sounds squashed on the TC-750LC after he added a variable resistor at the output, with no buffering. That volume control is affecting the input impedance of the following pre-amp the TC-750 is plugged into, and with that, the frequency response.
Out of interest, the TC-750 designer used to build me one off samples of my prototype designs, as he owned a small scale PCB factory.