I had both of 684's and 683's. Roksan Kandy matched with them. I can easiliy say that, difference between the 684 and 683 is not only bass(least important difference). In 683 detail was superbly well. Mid Range dirver is same with 8xx series. This midrange driver creates teh whole difference. Clarity, wide stage, pure music etc.... Also 683's have very strong bass drivers. Of course this is a difference between 683 and 684 but this is not the real difference between two of them. 684's also have a strong bass, but midrange driver also creates the midrange bass and it is ruining all musicality. Also 683' are far more coherent than 684's. I do not say 684's are bad speakers, they are good, but 683's are superb.
And 683's are giving a superb performance at low volume levels. You do not need to volume up to hear better volume. If someone has this kind of problem, this is a problem about power. If you dirve 683's with a 40 watt amp yes you have to raise the volume, but if you dirve them something like Roksan Kandy, in the lowest volume you hear every detail.
Last word, you have to burn 683's at least 50-75 hours to get the real performance. After this they will sing....