I have the Krell 400e and previously I owned Krell KSA 100 and KSA 150 for many years.
The 400e has far better slam and bass control over the older Krell's, but also the quality is significant better, with distinctly more details in the low and mid-low regions.
Also the mid's and high's are far better and in my vieuw 400e gives a very balanced, natural, open and detailed sound without favouring undesirable one area above the other. Thus I find also the neutrality topnotch, which I can appreciate more and more over the time.
Said that the 600e is somewhat but nevertheless clearly more dynamic than 400e, but the latter is already very good in this respect.