Nice Stereo System. Hard to justify the word "upgrade" from those Krells. If they are working to spec., it will be difficult to improve the sound. Those amps were the grail for many people when they came out. If you are bored with the sound, then, you could try tube mono amps and start a whole new phase of tinkering. I am not sure I believe all the rave reviews of the new Class D mono amps. If you have other reasons for making the change, ie. size, weight, heat, cost of electricity, age, etc..., then, you have many options....
Note: The way things are going, Class A looks to be going extinct in the future... If it was me, I think I would have those Krells checked out for a possible recap and bias and call it a day.