Had a quick look at this because its an interesting problem. Impedance is an indication of required current flow. A low input impedance is telling you the driving source needs to be capable of good current output. Otherwise if it doesn't have enough current you hear things like weak bass or a less sparkly dynamic sound.
That's not what you have. What you have is just plain not enough volume. This is a voltage problem. The spec you want is the voltage needed to drive full output. Then if you know that and can measure that much voltage coming out of your preamp then you know you have a problem with the amp. Otherwise if the voltage isn't there then you were right the first time and you need a step-up transformer to increase the voltage. The pics on Amazon show what looks like it might be an optional transformer, can't tell, but seems a clue to me.
That's not what you have. What you have is just plain not enough volume. This is a voltage problem. The spec you want is the voltage needed to drive full output. Then if you know that and can measure that much voltage coming out of your preamp then you know you have a problem with the amp. Otherwise if the voltage isn't there then you were right the first time and you need a step-up transformer to increase the voltage. The pics on Amazon show what looks like it might be an optional transformer, can't tell, but seems a clue to me.