I've done this myself and the hum is unlistenable, any advice would be much appreciated.
To give you concrete advices we should know (and better - see) your build.
There are several "universal" advices from other people in the forum:
1. The transformer and the PCB should be divided by a shield and be as far as possible from the input connectors.
2. ALL AC wires should be either twisted or twisted and shielded.
3. The signal wires should not cross AC power wires or (if it is impossible) cross it at 90 grad.
4. There should be only one grounding point and NO "spare connections". As other in the forum already pointed, the main mistakes are:
- not isolating RCAs "negatives" from the metal plate where they are mounted in.
- connecting RCA negatives to each other (and the single wire to the ground). Every RCA should have ITS OWN ground wire, connected to the central ground point.
- using metal standoffs (grounding the PCB in five point then).
- not connecting the ground wire from the power outlet to the system single ground point.
5. Use shielded or twisted signal-ground connections (including external interconnects).
6. Allocate the unit as far as possible from external sources of electromagnetic fields.
7. Checking "cold solder joints" in power supply (especially filter capacitors), checking heater circuits.
I'm not pretending to full coverage of factors, but IMHO point most frequent.
