I have actually built external crossovers for my Renaissance 90 EMIT tweeters using Mundorf SIO caps and copper foil inductors. There was quite a jump in efficiency and I had to slightly alter the tweeter amp gain to compensate. For single amp drive you could pad the EMIT with a slightly bigger resistor (0.5 > 1 ohm).
The main benefit of the upgraded crossover was improved dynamics. I also modelled the crossover frequency response in LT spice and found that there was a 2.5dB peak at 2.3KHz. I was not surprised to see this because I had always thought that they were a bit too forward in the presence range. I have heard that there were changes made to the crossovers during the Ren 90's lifecycle that corrected this but I'm not 100% sure. However, the Ren 80's are using diferent values and do have a flatter crossover response when modelled.
I subsequently modded my crossover values to achieve a response +/- 1dB from 1Khz to 20KHz. To do this correctly you need to change a 33uF cap in the EMIM crossover to 30uF and also insert a 0.22R in series with the cap. When I did this mod I also noticed that the 10 year old 33uF electrolytics had expanded slightly at each end. I replaced them both with Blackgate 30uF 50V NPO electros and there was an immediate audible improvement. The MF seemed more pure and imaging was much more accurate. The other fine tune necessary is to add a 1uf in parallel with the 5.6uF pp film cap in the EMIT crossover. Both of these changes combined will result in a much smoother sounding upper mid range and a far more seamless transition from EMIM to EMIT.
More recently I have disconnected the EMITS and am using Raal pure ribbon tweeters which sit on top of the Ren 90's. This also necessitated some fine tuning of the crossovers but the result is really stunning. I have also designed a DC bias system for the tweeter crossover so that both SIO caps are voltage biased at 33V DC.
The main improvement from the Raal tweeters stems from the much wider HF dispersion and far more extended treble, but they also seem cleaner, less grainy and sweeter sounding, but then again they are also very expensive. IMO the EMITs used in the Rens are a very good tweeter and with the 2 component changes mentioned above the response will definitely be flatter.
The main benefit of the upgraded crossover was improved dynamics. I also modelled the crossover frequency response in LT spice and found that there was a 2.5dB peak at 2.3KHz. I was not surprised to see this because I had always thought that they were a bit too forward in the presence range. I have heard that there were changes made to the crossovers during the Ren 90's lifecycle that corrected this but I'm not 100% sure. However, the Ren 80's are using diferent values and do have a flatter crossover response when modelled.
I subsequently modded my crossover values to achieve a response +/- 1dB from 1Khz to 20KHz. To do this correctly you need to change a 33uF cap in the EMIM crossover to 30uF and also insert a 0.22R in series with the cap. When I did this mod I also noticed that the 10 year old 33uF electrolytics had expanded slightly at each end. I replaced them both with Blackgate 30uF 50V NPO electros and there was an immediate audible improvement. The MF seemed more pure and imaging was much more accurate. The other fine tune necessary is to add a 1uf in parallel with the 5.6uF pp film cap in the EMIT crossover. Both of these changes combined will result in a much smoother sounding upper mid range and a far more seamless transition from EMIM to EMIT.
More recently I have disconnected the EMITS and am using Raal pure ribbon tweeters which sit on top of the Ren 90's. This also necessitated some fine tuning of the crossovers but the result is really stunning. I have also designed a DC bias system for the tweeter crossover so that both SIO caps are voltage biased at 33V DC.
The main improvement from the Raal tweeters stems from the much wider HF dispersion and far more extended treble, but they also seem cleaner, less grainy and sweeter sounding, but then again they are also very expensive. IMO the EMITs used in the Rens are a very good tweeter and with the 2 component changes mentioned above the response will definitely be flatter.