I bought a Moon P5 & was running them with a Bryston 4B-ST, the sound was very harsh specially in the treble region. Before that I ran my Bryston with a Yamaha RX-V800 Rec acting as a pre-amp and providing power to the center & rear channels. I guess the Moon P5 exposed the slightly brighter sound of the Brystons, even my wife didnt like the Moon & Bryston combo.
After reading some stellar reviews on the JC1, I went & bought a pair. The sound was way way better than the Bryston, very detailed & smooth at the same time. It doesnt have the bass slam like the Brystons, but who wants only bass. Brystons dont even remotely sound close to the JC1's, nothing about the JC1's are even close to a Bryston sound!! I have never had Krells so I cant comment, but from what I have heard in demos & friends places, they would sound closer to Brystons than JC1's.
It all depends on the whole setup & synergy also, but compared to Krell the JC1's would be definately more relaxed, though that doesnt rule out the fact that I wouldnt consider a Krell in the future. I like my sound fast with bass and accurate & dont prefer warm tube like sound & for the moment the JC1's do everything right. You do have to burn them in and make sure use the balanced connection. Also proper power cables make a difference on the JC1's