Connectors soldered on the wrong side
Do not try to save the connector! This will very likely damage the PCB which cost 100 times more than the connector.
Instead, split the connector into pieces with cutting pliers until you are able to remove the pins one by one.
Next empty the holes with a de-soldering pump (this one works great : Jonard Industries DP-100).
IC's in the wrong place
There are 3 different kinds of 2x4 pins IC's: Don't mix them up!
They usually don't like being powered in the wrong place and are likely to die.
Conductive solder flux
Some solders leave a slightly conductive flux residue on the PCB. This will mess the side chain circuit which works at very high impedance. The symptom is a meter that doesn't stay at 0 with no input signal. It can stay at the right or at the left of the 0, changing slowly.
The solution is to completely immerse the 3 PCB's in isopropyl alcohol during 10mn at least, shaking and brushing the surfaces. A simple clean without immersing won't be enough.
You should check your solder before committing. Put the 2 probes very close to each other on a spot without any track but with some flux residues. Set the ohmmeter to megohms. If you get any reading then it is not good.
One recent example of solder which had this problem was the WBT-0800 with halogen free flux.
