Vintage equipment, up through the 1960's or 1970's, virtually always derived the headphone outputs from the main outputs - through dropping resistors in the 100 - 250 Ohm range.
This offers very little damping to the headphones, and can also result in a significant variation in frequency response due to the interaction between that output impedance and the impedance of the headphones.
Because of this interaction the actual frequency response, and the variation from flat, will depend on the impedance curve of the particular headphones you connect.
As such, it delivers a sort of characteristic sound, which is quite popular with some fans of vintage equipment, and some people actually purchase vintage receivers so they can get the vintage headphone outputs on them.
If you look, for example, on Head-Fi, there are whole discussions dedicated to "using vintage gear to drive headphones".
(At the time, it was generally done that way to cut costs, because high quality low cost small IC amplifiers weren't yet available.)
With the resistors in-line, the A-100 will mimic this performance, with most of its pros and cons (although the main amps on the A-100 are a lot quieter).
If you install the jumpers and bypass the resistors the A-100 will sound like a modern high quality headphone amplifier.
However, as a consequence of its power capabilities, it will be "dangerously powerful" for low impedance headphones.
Note that this is NOT a concern with high impedance headphones, including 300 Ohm and 600 Ohm models, which are more properly matched to the higher output voltage.
With 300 Ohm or 600 Ohm headphones, the output level with the resistors bypassed will only be a few dB higher, and you will receive the benefit of better damping.