"Set at" as in the output goal or "Sensitivity : 98dB/1mW?"
Because given a loud enough input signal to work with you only need 1mW to hit 98dB (note that gain and input signal voltage can vary that), and then you only need 19mW++ minimum reserve power to hit 113dB peaks assuming you are hitting 98dB with just 1mW.
As much as conditions for input signal voltage and gain, plus ambient noise, are not always perfect, actually listening at 98dB is going to hurt, and whether the gain is low enough is probably a more likely problem when the high sensitivity headphone is within the impedance range where the amp kicks out most of what power it can produce.