I don't recall seeing this issue raised before about the 007A. The other day as I was listening to my 007A (Serial # SZ2-1286) I noticed that if I tapped the metal bands in the arc assembly (as Stax calls it) I could hear a low frequency ringing for a bit over a second. Now I have always thought that resonances were the enemies of good sound, especially in speakers and headphones because they can be excited by similar sounds being played and artificially enhance those particular frequencies. In other words they cause coloration. Generally you expect that if you tap a good quality speaker you will just get a dry thwack.
So it seemed that if I could hear a ringing from a tap on the assembly, while wearing the headphones, this could indicate a resonance issue when the phones are being played.
I have always thought the 007A had a muddy bass. There was considerable discussion about this problem shortly after this model came out as there also was for the original 007 which was criticized as being "dark" sounding. I adopted the "Spritzer mod" of flattening the spring that pushes the earpads out. And that does seem to help this problem a bit.
Interestingly when I put my finger across the 2 metal bands the resonance seemed to stop. Today I experimented putting a crude linkage across the 2 bands with 2 parts of a cut-up old credit card held together by a nut and bolt and it seems to dampen the resonance.
I am still listening to this set-up but my initial observations are that it does reduce the bass and make it drier and give a better attack to some sounds. The photo shows the mod
I assume that Stax has used this same metal assembly on all the 007's, i.e. the original, the 007A aka mk2, and the more recent model which some people call the mk2.5. I think mine is the original A or mk2 design and I recall buying it within a year of the release of the 007A.
I notice that the 009 has a similar looking double metal band except that the two strips appear to be held together (by plastic?) Since I have not actually seen one in person, I can't say for sure. Possibly this is not just a cosmetic change but that Stax realized there was a problem to be addressed?