Just installed the C4S upgrade and took 'er for a test listen. Listened to two of my favorite test songs, Broadway Hotel and Year of the Cat from Al Stewart.
The sound is great... crisper is the term I would use to describe the improvement. Everything seems "tighter" if that makes sense.
I think I am pushing the limits of what my Yamaha receiver can do though.. so I guess a S.E.X. is next!