Dillen:
It could be a problem with the crossover filter too. The capacitors dry out with age.
A capacitor kit exists for this model and I may be able to help you to a good
used original midrange driver.
Martin
I'm sure the drivers and crossover elements are different in your S120s, but I had the same problem with my S45-2s: distortion in one midrange only, and only at listening levels louder than typical background listening levels.
Martin supplied me with capacitors and a new midrange, and in the end, it was the crossover that was the cause. Three months later or so, I am using my original midrange and all is well.
Current primary listening: SMMC20EN -> BG4002 -> BM4000 -> Beovox M70