I just recently switched from the SE RCA interconnect to the SX RCA interconnect. I had tried several interconnects a few years back and in my system the SE was my favorite.
I had been hesitant to demo the SX RCA interconnect as I was really enjoying the SE but as most audiophiles do, had to try the SX interconnect.
I was kind of hoping there would be no difference but the SX RCA was clearly better than the SE RCA interconnect. As others have stated, the midrange was the most apparent change. Previously with the SE some female recordings would have a little extra sibilance and compared to the SX a little more energy was apparent in the upper mids, lower treble which sometimes could cause a little listener fatigue. This was very minor and I still really liked the SE and still consider it an excellent interconnect.
The SX complete eliminates this slight excess energy and I notice myself listening for much longer periods of time with the same material. The bass is slightly better and the highs even more refined. Soundstaging and dynamics which were huge strengths of the SE are very similar with the SX. The overall sound with the SX is smoother and a little more relaxed but at the same time slightly more transparent than the SE which almost seems not possible.
I still consider the SE one of the best interconnects in the $1000 range but the SX is clearly better but of course costs more. I compared the SX RCA to the Wywires platinum and High Fidelity CT 1 interconnects which are at a similar price point. All three were excellent and I could live with all of them but the SX's were my favorites.
I am using Green Mountain Audio Callistos as speakers.