There is a broad excess of energy in the range from about 900 Hz to 2.2 kHz... the biggest issue was diffraction, which caused the elevated response in the 1-2kHz range. I addressed that with an anti-diffraction frame
they have an interesting series crossover between the tweeter and supertweeter, BTW). ... Shorting out one of the resistors in the woofer circuit gave increased attenuation at high frequencies, and an increased Q, which was also needed.
All of this was documented on this site:
https://www.avsforum.com/forum/89-speakers/3068362-improving-sony-ss-cs5.html
This was more interesting to me than most all of the debate over whether the speakers are junk or pretty decent and an exceptional value. As for that question, I agree with most others that the choice of drivers and crossover points is inexplicable, but that nevertheless they are a good speaker and an excellent value.
But back to your mods. I'd be very interested in seeing the crossover schematic both before and after your mod, and including that series crossover for the two tweeters. In a subsequent post of yours you showed the individual on-axis responses and concluded that the the super-tweeter contributes very little below 10 kHz. But perhaps things are different in the off-axis response? I.e., it might be that the tiny tweeter has substantially better dispersion, such that in the listening window response or whatever, its contribution is much stronger than what your graph implies, possibly even stronger than the bigger tweeter depending on the off-axis angle.
Your anti-diffraction mod is interesting. You provided response plots to show the before/after effect. You did this for both the crossover mod and the anti-diffraction mod, and to give you the benefit of the doubt, you would have done these measurements with just one or the other of the two mods, such that you avoided confusing the effects of the two mods.
Diffraction occurs over a broad range of frequencies starting when the wavelength is small relative to the enclosure or the baffle. However the 1st diffraction peak occurs at a fairly well-defined wavelength where the wave propagated at the corner is in phase with the direct wave. The wave propagated at the corner is a wave associated with a "soft reflection", where a 180-degreee shift in phase occurs. Thus, the longest wavelength at which constructive interference can occur (i.e., the first peak in the diffraction ripple) is the wavelength at which the distance from the center of the driver to the edge of the cabinet or baffle is equal to one-half wavelength. In other words, the 1st peak in the diffraction ripple, for a driver centered horizontally on the baffle, occurs at wavelength matching the width of the baffle. In this case, 7", which corresponds to frequency of roughly 2 kHz. Some reinforcement will occur below this frequency and some above, however there just isn't any way that the sharp response peak at about 1.1 kHz can be attributed to diffraction. The broad elevated response that you described, from 900 to 2.2 kHz, is almost certainly a feature of the driver itself. Diffraction may contribute in the upper half of that plateau, and at 2 kHz especially, but this is the upper end of the plateau.
I recall reading a few years back the writeup that Linkwitz did on baffle diffraction, which is probably still on the site (it is still active). He had something to say about the effectiveness of rounded edges on cabinets. I read through it too quickly and this was probably more than five years ago, but on the off chance that I recall what he wrote, the rounding starts to become effective where 1/4 of the wavelength is shorter than the radius of the round-over. Since you used a pipe with 4" diameter (approximately), this implies that the longest wavelength at which it would be even modestly effective should be 8", or about 1.7 kHz. Of course this effect is a gradual thing, but the point is that rounding the corners doesn't have much effect until reaching higher frequency where the 2nd peak, 3rd peak, etc., of the ripple are found, but not the 1st peak, which is always the strongest and the only one that can't be neutralized by placing the affected driver 1/3 of the distance from one edge to the other edge. Thus, even if the sharp peak at about 1.1 kHz or the broad elevation from 900 to 2.2 kHz were genuinely diffraction effects, the mod you did would almost certainly not have any significant effect at these frequencies.
I am curious to see the crossover schematic both before and after your crossover mod.