recabling my Stax sr-009 to pure copper cable

Try playing your headphones at very high volume (louder than you would want to play them if they were on your head) for a few hours.

I've found that my older Stax headphones sound similar to what you are describing if they are not used frequently and/or given that kind of workout from time to time. That has been the case ever since they were new.

The fact that the symptom is not exhibited by different Stax models which are copper cabled does not by any means necessarily indicate that the cable material is responsible for the difference.

Regards,
-- Al

By the way, silver is MORE conductive than gold. What gold has going for it as a conductor is that it doesn’t easily oxidize.

+1. Both silver AND copper are more conductive than gold, and by a considerable margin. See the resistivity and conductivity tables that are widely available for various metals, such as the one in this Wikipedia writeup.

That being said, in the case of a home audio system it can be shown analytically that under most circumstances whatever sonic differences may exist between cables made of these metals are not due to differences in the resistance and conductance of the metals, assuming the cables being compared are of equal gauge and length. A possible exception to that being speaker cables, in situations where cable length is long, speaker impedance is low, and wire gauge is narrow.

Also, to add some perspective, simply making a copper conductor one gauge size larger will lower its resistance by well over twice as many percent as it would be lowered by changing the original copper conductor to a silver conductor of the same gauge.

Regards,
-- Al