recabling my Stax sr-009 to pure copper cable

How do you know it is the wire that is the source of your concern? Have you heard a set with different wiring? Have you heard other Stax electrostatics and compared the sound?

I have only heard the 009s once, at an audio show where it was powered by a Woo amp. I own a Stax Omega II Mk 1 headphone and a Blue Hawaii SE headphone amp. Because I heard the 009 in an unfamiliar system, I cannot really compare the two. I have read the comments of others who tend to agree that the 009 sounds brighter. Some actually like the additional top end lift of the 009. To me, the Omega II's are themselves a bit too bright, and they are extremely merciless when it comes to matching upstream components.

If you don't like what you are hearing, it might be worth trying a cable swap, but, there are a whole lot of other ways to alter the sound as well. The easiest, and usually most effective, is to swap tubes (assuming your amplifier/interface is tube based). I would next look into the interconnect cables to the amplifier. Even power cords matter too, so that is another place to investigate.

Good luck on your quest.

SnibelSnabel: You are not hearing things, IMO. Copper is definitely warmer than plated wire or some silver cables. I use an old pair of Cardas Cross cables in my headphone system. It does add a euphonic coloration to the sound however.

Generally speaking, my experience has been that silver, being the second best conductor we can use besides gold, is one where silver will improve performance across the board on all levels. Initial 'tinniness' will always occur in cables with silver because I suppose it's some kind of chemical trait. This trait will give way later on after the cable has fully broken in, but this takes time. TRANSITIONING TO COPPER IS NOT RECOMMENDED because copper is completely inferior as a conductor. The 009s are known for a particularly sharp sounding signature, so I'd personally recommend either waiting longer until after the headset has broken in some by playing music on it for long periods as others have suggested or completely eliminating the problem by getting a tube energizer to fuel the cans. The latter is money restrictive, but would definitely provide a definitive solution. Whatever you choose, I would strongly recommend sticking to the silver cables, as Stax is one of the few companies that understands the value of good cabling on their products. There's a reason they don't allow interchangeable cables on their sets. 

Try a different driver amp!

Do not screw with the cable!

I have now heard the 009s on my amp (Blue Hawaii SE), and it is somewhat on the bright side and not quite as good in the midrange as the 007.  I can see why someone might be tempted to change the wiring, although I have no idea if that will work out well.  I am still considering an 009 because it has better bass response and dynamics, and does not require the amp to work as hard (more efficient).

For speaker wiring, I have found that silver wires sound more extended on top, but, when done correctly, silver wiring is particularly nice (don't know about silver-plating).

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.