Insulator on the pick up sensor inside a Bosch hall effect distributor is difficult to see unlike the Marelli unit, which can be seen with ease when you take the cap off.
The two wires loose the covering and you are left with nice green/blue exposed wire.
The way they work is identical but Bosch have made it a little overcomplicated.
OK so you have tested the distributor with a multimeter?
That is connected it to the pins in the plug and spun the engine over to make sure you are getting a pulse - this is just to make double sure that the distributor can be ruled out.
Check the three pin plug on the harness also - black is earth (this earths the plate the amplifier sits on also so make sure its doing its job) , double Grey is positive, blue/white goes to the rev counter so ignore this.
Also remove the sheath on the amplifiers plug and check for broken wires (sometimes the pins pull out) any bent pins on the Bosch amplifier box?
Do the same also for the distributor plugs - just to make double sure!
Is the ceramic resistor fitted in line on the positive side of the coil - if so bypass for the time being to make sure you get 12 volts on the plus side of the coil.
If the above checks out then your mechanic is 100% right and not 99% its the amplifier that's faulty.
Seems odd though that you have purchased two replacements and they also don't work - makes me a little suspicious of your wiring harness.
Can sell you a guaranteed working unit, if you fit it and it does not work, chances are the fault is on your car.
Did the car run and has developed this fault or did you buy it as a non runner? Just curious.