Too much guesswork, if you ask me. It is a very ineffective method. And why would 2 capacitors of good quality be defective? As far as I know nobody has complained about unreliable component delivered with the LC:
You did everything right in the beginning, measuring the supply voltage and other important voltages, plus checked the orientation of components etc. Just continue with simple and logical measuring, and you will find the fault very soon.
1..Set your multimeter to measure small AC voltages. 2. Send a signal to the head-amp, preferably a sinus tone (you can download test tones, or a free tone-generator for PC like ’Sigjenny’). 3. Now measure, say, at the input and the output of the op-amp etc. Is there amplification?
Remember to set the dip switch to amplification, and to turn up the volume..
AC measuring of the signal will not destroy the amp. (one must be more careful when measuring resistances, because a current then is sent through the circuit).
You are aware that electrostatic discharge easily can destroy the sensitive input section of an op-amp? Both your op-amps may be defective for the same reason, if the problem boils down to the op-amp. At least it can not be excluded. (Never mount an op-amp without touching the ground of the amplifier with the same hand before the pins are making contact, and moist skin is an advantage).