I have the 2.4s and they are wonderful speakers. I can't make up my mind as I switch back and forth, depending on my mood, between CJ CAV50 tube and CA200 solid state control amps. I tend to favor the CA200 simply because it has 185 WPC and provides better top and bottom extension and dynamics, but there are times when those tubes provide that certain sweetness. They are not hard to drive but do respond to high quality, high power and I agree the Krell 400cx would probably work very well. Now if I can just get off my butt and hook this pairing up I'll comment on that set up too. After 6 months they are still breaking in and getting better so please be patient with them regardless of what you settle on for an amp. The bass especially has gotten much better, fuller, clearer and more extended.