A lot louder” likely means at a given volume control setting, which is meaningless.
For the whole chain the key elements are:
Headphone/Speaker amplifier – Output voltage is controlled by volume control.
Energiser – Output voltage is controlled by the driving Amplifier AND the step-up ratio.
We deliberately use a low step-up ratio (1:16/1:32), as with increasing step-up ratio transformer quality becomes more compromised with increased distortion and limited frequency response.
The original Stax energiser adapters are rated to produce full output from a 5 Watt / 8 Ohm Amplifier (or 6V), suggesting a step-up ratio of 1:48, so naturally, they will be much louder at the same volume setting on the HP Amplifier.
Hence swapping between the iESL and any other “Energiser” is only valid if the volume control is corrected for the difference in step-up ratio.
To consider this as car alternative, Stax and iFi Energisers are like gearboxes with different gearing ratio, so even if the same engine the gas pedal will need different degrees of being depressed for the same speed.
