You say "-115 is the threshold of hearing. we can not receive anything below 115 no matter what"This is based on reference listening level of 110 to 120 dBSPL. BTW, it is trivial for a headphone and amp to generate 120 dBSPL. I test at 114 dBSPL in all of my headphone measurements.
"no matter what" is misleading.
The threshold of hearing can be as low as -5dBSPL (or 0dBSPL we can argue about this).
-115dB Corresponds to about 0dBSPL if 0dB corresponds to Reference listening level of 110 to 120 dBSPL.
As your talking about an powerful headphone amplifier... its 0dBrA point is not necessarily based on reference listening level of 110 to 120 dBSPL!?
So gifting sensitive IEMs and a powerful amplifier 0dBrA from the AMP could be way over 120dBSPL.BTW, it is trivial for a headphone and amp to generate 120 dBSPL.
so -115dB could be over 0dBSPL
Even if no body would listen at this levels Lots of people want there amp to have High gain and headroom.