Please provide details about setting the loudness / volume control on the KSE1500.
The KSE1500 earphones must be used with the provide amplifier / electronics package. The KSE15000 maximum SPL (sound pressure level) is around 113dBSPL; this is the overall loudness of the system before distortion occurs. The total obtainable SPL is a function of the input level of the signal provided to the KSE1500 and of the amplification provided within the KSE1500.
If an audio track is quiet, one might increase the volume toward the maximum setting of 25. If an audio track provides a robust signal on the KSE1500 input meter, one will likely be using a setting of around 17 or 18.
Note that it is not recommended to drive the KSM1500 from another headphone amplifier.