I just recently aquired a used Krell KAV-300i, and I can say that it is an excellent piece of kit. Although I didn't really try driving the Sequels with it to any volume (just hooked them up for a few hours to the amp posts and did some low-level listening) it sounded VERY clean, neutral and precise. And as a preamp, it is VERY nice. It's versatile, dead silent, and it's precision is a good match to the rest of my system...
The KAV-400xi that you are looking at is th enext model up, and a much newer version than mine (my 300i has the "old style" Krell logo etched on the facen, not the new "arrowhead KAV" logo). So I imagine it should drive your speakers very nicely.
You've got to remember, Krell watts are a lot like Carver/Sunfire watts. They might be rated at 150 or 200wpc, but thise are VERY high-currrent output watts. 10 Krell watts are like 30 Luxman or Adcom watts, and like 100 Sony or Onkyo watts. Unless you are playing Telarc's digital 1812 Overture at performance levels, you should never have to worry about running out of juice with that Krell...
Look for one used if you don't mind used gear. Sure, it won't have a warranty, but Krell gear is ASTOUNDNGLY rugged, and has Nasa-Grade build quality. Unless it's been SERIOUSLY abused, used Krell gear should work great, last a LONG time, and it comes with the added bonus of saving you 50% or more off the new retail. Take the saved money and buy better cables...
--Richard