Okie dokie, so I did a shoot out between the serial numberless RS-1 and the new, buttonless RS-1 last night. The serial numberless RS-1 is reterminated in balanced neutrik xlrs, so I used a black dragon xlr->1/8 with it.
I used my two favorite test tracks, both Genesis of course: "Squonk" from the diament mastered atco japan ATOTT, and "Invisible Touch" from the black triangle IT.
After repeated switching back and forth listening to the same portion of one of the above songs over and over, I determined that the bass is very similar, and the treble is definitely the same. The only potential difference, which is so subtle that I admit it could be pure placebo, is that the midrange is slightly softer and more "liquid" in the older model. Or this could just be because the newer model has a tighter headband.
In conclusion, there is no significant difference between these two phones.
Always keep in mind that a wonky tube blew the left driver of my old RS-1 a year ago, and I sent it in to Grado Labs for repair. I have no idea what they did to fix it, but when I got it back it sounded exactly the same to the best of my memory.