Update. All parts checked out. No diode problems, cap problems, or resistor problems.
All grounds check out
I taped all power wires to run right against the case. repositioned signal lines, etc.
I have to admit, the buzz is less noticeable if I use a 4 watt per channel SE amp. If I use 50WPC to 350WPC tube amps, the buzz is haunting. If I use the Zbox on 130WPC to 160WPC SS gear, is just as haunting.
So things to try next.
-using two 100 ohm resistors to ground each leg of the heater supply
-reversing the phase of the heater supply
-converting heater supply to DC
-choke addition to power supply
-add more capacitance to power supply
-scrap the Zbox and clean up the wiring rats nest by using a PCB board for a Zkit4 or maybe Zkit5 and just keep the box around as a buffer (I was more interested in the Zbox, but can't spend too much more time on chasing this ghost).