I had the 601 S3's for a while and thought that were great little speakers. I think the complaint that some people had with the S3 range was that they can be a little bit bright.
I haven't had experience with the 685's, but I've currently got the 683's and I'd say there's definitely and improvement on midrange detail and overall sound. I don't know whether that's anything to do with the FST driver in the 683's though which I don't think is present in the 685's.