Oh I thought the soar had a 600ohm headphone amp
Actually 600 ohm headphone would be one giving good electric damping factor with 100 ohm output impedance and 250 ohms would be decent.
Now if modern consumer headphones were in that impedance range...
But typical modern consumer headphone with 50 ohm and under impedance have it entirely the wrong way with ratio between headphone impedance and output's impedance.
Meaning there's very little electrical damping and it's mostly up to mechanical damping of air and diaphgram material to damp driver's unwanted movement.
But mechanical damping is always compromise, just like suspension of cars trying to keep tires on road while smoothing car's movement:
Stiffer shock absorbers and it's bumpy and harsh drive, because suspension can't follow road surface well.
Loose shock absorbers and it's bouncing jelly like drive.
Electrical damping can be visualized as pendulum moved around by some kind motor:
Pendulum's mass resists pushing and pulling, except at resonant frequency where pendulum starts to swing easily and wants to keep swinging.
Low damping factor is similar to motor being too weak compared to pendulum's mass and having problems pushing and pulling it.
High damping factor is similar to strong motor, which has no problems in controlling pendulum's movement accurately.
For some reason Asus has always had problems in that part of electrical engineering starting from Xonar D model.
Maybe that's their way of short circuit protecting output just in case someone sticks that good old fuse replacement called iron nail into output...
Or more likely circuit design fails to stay stable with every load without that big "speed limiter".
Just like TPA6120 based headphone amps usually have at least 10 ohm output impedance, because of its slew rate being more fit for AM radio transmitter than audio amplifier.
It's kinda "pick your incompetence" situation.