I have heard the C-1s, and overall don't recall any significant improvement. Sometimes the older IS the better, which is why I referenced W.E. tubes.
Mfgs are always going to "upgrade" their products for sales purposes, right? If the Extrema is a collector's item as other speakers, tubes, etc. could it that the originals have the most value or quality? I'm not in the business, just a hobbyist...