I owned 600c and 700cx. The 700cx was bought new, it was not the 600c upgraded. The 700cx is a little darker sounding and it is also smoother. The 600c in direct compairson, can be a little harsh in the edges of voices etc, a slight sting in the treble if compaired to 700cx and with equiptment that proves that. I used the 700cx and 600c with Teac P-70 and D-70. The 600c was connected to PMC MB-2. This combo had a hard tone. When i switched to 700cx the top end was significantly smoother. If you use 600c, you can also use a smoother preamp than Krell. The 600c is a very very good amp though. Also, the Teac and Esoteric combos is so incredible detailed in the top end. They will not do a romantic sound. A smoother cdp, like krells old 25sc, is way more forgiving. Though, you will miss all of that the TEac combo did. Only thing the 25sc did better was bass slam. I also used KCT and KCT 28c cdp.
If you use ordinary xlr at the 600c, use cables with smooth top end. A cheap cable that is very good is signal cable silver resolution xlr. Very good interconnect for that price. We did a blind test. Silver Res, compaired to Transparent Audio's cast cables. No one out of 4 person could differ the two. All was very deer Krell users.