I did the deed over the weekend; I reterminated my RS1x with an XLR balanced connector. Since I didn't change the cabling, this was a good way to listen to the difference that balanced brings vs. single ended.
As some folks have said, this was relatively easy to do, and only took about half hour total - and for perspective, my soldering skills are pretty basic. You must use a multimeter to determine which wires should be soldered where though, so that you don't blow your amp or headphones. The most complicated part was that Grado had to make things difficult by using 4 identical blue wires - two per channel. Mine measured 40 ohms when wired correctly.
FWIW, even just a soldered termination needs break-in time. I didn't expect this. In my initial listening, I heard a lot of glare, minimal bass, and simply could not hear details that are expected in my audiophile reference song list. The sound improved after about 10 hours in, but until then I had some panicked "OMG - what have I done???" moments.
TL;DR: balanced improves everything that the RS1x is already good at and especially mitigates its weakness. Specifically, the RS1x was already open, textured, and layered, but with XLR, these traits stuck out all the more. The soundstage didn't necessarily open up, but the images became more stark within the layers. I would chalk this up to higher resolution. If you appreciate these traits like I do, this will put quite the smile on your face.
As an example, Fleetwood Mac's Dreams is one of my reference songs. I have heard it more times than I can could count, but I had never heard Stevie Nicks faintly humming the song's melody at about 12 seconds in. Nor have I heard Mick Fleetwood's drums have as much texture as they now have.
I didn't hear any difference in tonality.
Bass definitely improved. This had been the RS1x's biggest weakness. I was actually able to bump up the bass via EQ another 2 decibles - which made a meaningful difference. I now don't feel like I am missing anything across most genres of music. This probably still isn't the best headphone for Synthwave or Downtempo Electronica, but the lack of bass no longer stands out (e.g., Thievery Corporation sounds fantastic). But the RS1x survived torture tracks such as Claptone's No Eyes. And I was able to hear sustained sub bass on tracks such as Sonique's It Feels So Good (at 1:36), or Mesa from the Bladerunner 2049 soundtrack. Previously all I got was distortion without actually getting the sub bass. Bass impact was also improved - e.g., the drums at the beginning of Joy Division's Atmosphere have never sounded more jarring.
My experience with balanced amplification is that you get the effect of greater volume, but without the sound getting louder. This allows me to hear deeper into the music. The only downside with the RS1x is that I prefer a more distant presentation, but now the perspective sounds a bit physically closer to the performance.
If you already have a balanced amp, I highly recommend this mod. You can always use an inexpensive adapter to plug into single ended sources. In terms of ROI, this was $12 (from Amazon) very well spent. I don't think I have ever heard this much sonic improvement for such little money.
I am still tempted to have a 3.5mm jack installed on each headphone so that I can use my aftermarket XLR cables. My experience is that with higher end cables (more complex geometry, better metal such as UP OCC/single crystal copper, etc.) you get even more of a boost in soundstaging, imaging, layering, and textures. But for now I will just enjoy the boost I am getting...
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