If you still have significant hum after you fix those problems, you probably have a crap power transformer and the extra hum induced on the power rails that pass right beside it is pushing the first amp stage (the rightmost board) into oscillation, or at least is bleeding through into that amp stage.
Remember that any hum introduced in that stage is amplified at every step along the way, so it's really better for the power to go in on the other end so that by the time it gets to that stage, it has gone past multiple capacitors and has been smoothed out further. This is one of the disadvantages of dealing with real-world electronics—sometimes you have to do things backwards to get the right results.
Thus, the cure that worked for mine was removing the power wire from the right amplifier board and adding a similar molex connector onto the leftmost jumper board. The pin spacing on the molex connector is identical to the trace spacing, so the only thing hard is drilling the holes in the centers of the traces. Beyond that, it's just a matter of polishing of the sealant with a fine grit sandpaper to expose the bare copper traces, then soldering to both sides of the trace.
Be sure to check conduction with an ohmmeter to make sure each pin on the molex connector is hot to both sides of the jumper board and to make sure that no trace shorts to any adjacent trace. Do this *before* you reinstall the jumper.
Also, be sure you get the orientation of the connector right. You don't want to end up reversing the order of the pins accidentally.
It's fairly easy to trace the wires—they basically go straight across with the exception of the ninety degree turn on the rightmost board.
You should be able to get the connector at any decent electronics parts store, though you may have to do what I did---buy an eight-pin connector and cut off one pin with a pair of scissors....
