If the wires to the tank are reversed, there will be no reverb at all. The spring driver in the tank has an impedance of 8 ohms, while the receiver in the tank has an output impedance of several thousand ohms. Therefore, reversal will cause a complete no reverb condition.
Drive is surely important, but just as important is the preamp circuit that amplifies the spring signal. Just as with your 60's driving experience, the same is true of this unit as well. A good rap on the spring tank should produce a noticeable racket from the speakers. If not, the preamp circuit should be investigated for proper operation as well.