The xo changes did not seem to find their way into an updated schematic but there must have been errata's sent out to distributors. The only schematics I have been able to find have all had the same values as those in my early gen 90's. The changes I am aware of are in the EMIM LPF (2nd pole) and there were also some updated values in the woofer XO to provide a bit more punch and speed in the bass - at the expense of a little extension. None of these changes make any radical difference but the changes to the EMIM do make the response slightly flatter through the crossover region and address a 2-3dB spike at 2.3kHz. Bear in mind that the values chosen in the EMIM / EMIT crossover are mostly all 5% standard values - which are easy to obtain and result in a lower production cost so there is actually some scope for fine tuning. If you look at the crossover schematics for the IRS Sigma EMIT / EMIM XO you will not find any standard value caps or inductors - hence the higher price tag. Fortunately though the solen caps in the EMIT XO are all very HQ and ditto the EMIM with the exception of C4. This cap is somewhat critical because any capacitance modulation here will result in some audible distortion, which is why I changed it to a BG. The Sigma's address this by using a polyprop cap in parallel with a HQ electro. This is a 50% improvement but I'm sure the Epsilon uses pp throughout.
In summary, the Ren 90's are using the exact same EMIT + EMIM drivers as used in the Sigma's and nearly the same quality of crossover components - save for one cap. The main difference seems to be that the Sigma's do have fine tuned component values for a slightly more linear response - but remember that subsequent revisions of the Ren XO have largely addressed these so if you are not aware of any colourations then any modding is probably not worthwhile. FWIW I have never heard the Sigma's but it's unlikely that they would offer any significant improvements over the Ren's.
In summary, the Ren 90's are using the exact same EMIT + EMIM drivers as used in the Sigma's and nearly the same quality of crossover components - save for one cap. The main difference seems to be that the Sigma's do have fine tuned component values for a slightly more linear response - but remember that subsequent revisions of the Ren XO have largely addressed these so if you are not aware of any colourations then any modding is probably not worthwhile. FWIW I have never heard the Sigma's but it's unlikely that they would offer any significant improvements over the Ren's.