THERE IS A SIMPLE SOLUTION.....
If you find that your headphones are so sensitive that you can barely move the Volume control on the A-100 off the bottom stop....
Or, if you find that, because you end up with the Volume knob so near the bottom stop, there is a slight but annoying channel imbalance....
Or, if you find that there is too much background hiss, AND THE HISS IS COMING FROM THE SOURCE (you'll know because the hiss will drop when you turn off or disconnect the source).
The solution is to "lower the gain structure of the A-100" by putting a pair of passive attenuators between your source and the inputs on the A-100.
For example, if you put a pair of 12 dB passive attenuators on the input, then you will end up turning the Volume knob on the A-100 12 dB higher to achieve the same output level as before.
This will "put the knob on the A-100 in a higher and more linear portion of its control range".
Note that, while this will not lower the noise floor of the A-100 itself, if the output of your source component has a noise floor, and that's what you're hearing, this may reduce it significantly.
(The result will depend on exactly where in the circuitry of your source component the noise is coming from.)
If you want to try this, I would recommend these (and they are quite economical).....
https://www.parts-express.com/harrison-labs-12-db-rca-line-level-audio-attenuator-pair--266-244