S5X 1.1 version is acoustically identical to the 1.0 version but with the USC calibration plugin included. This improves their flat frequency response from ~2dB down to ~1dB tolerance using DSP.
The S5X are specifically tuned for spatial audio mixing with a brighter character that helps with 360 positioning and movements of 3D binauralised audio (e.g., Dolby Atmos).
Regarding S4X vs S5X tuning, the S4X are slightly darker sounding and warm. They represent how a common room will sound. That, together with acoustics properties, makes them a staple in over 10,000 studios worldwide for mixing and producing in stereo.
Both are tuned flat for their purpose. It’s not one target frequency response curve that fits all use cases. Remember how Fletched-Munson curve demands different targets for quiet background music vs loud clubs. In a similar way, spatial audio demands different targets on headphones.
Regarding S5X vs S4R tuning, the S5X is open back model, made for mixing and is the opposite of the S4R model, made for recording. S4R tuning is made with clarity in mind. That is to deliver accurate monitoring for recording and podcasting sessions. Mids forward tuning is welcomed in monitoring for details and dynamics. Both of these are crucial when you’re recording instruments or vocals at the highest level. So not directly comparable with the S5X.