I have just bought a brand new Audio GD SA-31 (not SA-31SE) Headphone amplifier at a bargain price. After burning it in for more than 50 hrs I carried out an audition with three of my headphones, i.e. Audeze LCD-X (with Toxic cable Silver Widow), Senn HD800 (with Toxic cable Black Widow) and Hifiman HE-500 (with Toxic cable Black Widow). I was disappointed by the overall performance of this SA-31 as it is not ear friendly when compared with my other three headphone amplifiers, i.e. Yulong A18; Fidelity Audio HPA-200SE and Hifiman EF-5 (modded – see this link: http://www.head-fi.org/t/711004/hifiman-ef-5-headphone-amplifier-mods-and-rolls).
From my knowledge I have the feeling that it should not be the fault of the circuit design or the components being used. I suspect that the output impedance of SA-31 might be too low as I found it did not perform well with high impedance headphones. The music was laid back and though it appeared to have more space I found that it was unreal and artificial. The music was not at all three dimensional.
With that in mind I inserted two 33 ohm resistors in series to the L and R of the headphone output socket with a view to adjusting the output impedance. Well, as I have expected the result is really excellent. For best performance I put the gain to High and it performs much better than using Low gain setting. The music is cleaner, with better sound staging and with better musical separation. The artificial space has disappeared. I have tried Warm 1 and Warm 2 settings but IMHO I still prefer without any warmth being added. Now the SA-31 performs almost the same as Yulong A-18 and Fidelity Audio HPA-200SE which cost almost double the price.
I have uploaded two photos to show how to add the two 33 ohm resistors for reference. If you have tried it out you may wish to confirm if my finding is correct.