Philo, you'll like the zmod boards. I don't have the tone controls,but don't miss them. I did keep the blend switch for mono recordings as well as the balance.
If you haven't checked , be sure the solder on the phono inputs isn't touching the chassis.
I had a hum problem drive me nuts and that was the cause IIRC.i used some fiber washers to space the board, I think. Perhaps others can chime in.
In swapping gear around, I found there is some mismatch between my recorder and preamp. When the recorder is removed all is good. Connected ,the sound thru the speakers becomes muted and rolled off. I tried a buffer to no avail.
Perhaps that may be why I removed the recorder in the first place.
I feel kind of lost without it. Which leads to the next quandary.