Great speakers. Unfortunately, you'll just have to try a bunch until you find one the creates a good synergistic match and satisfies your personal taste. Me, I'm not of fan of Krell as I find them to be a bit on the hard, analytical side, especially paired with Nauts. However, I prefer a bit sweeter, more musical approach such as the classic ARC sound.
I understand the Nauts can present some pretty wild impedence swings so you'll want something with some gumption to it. Naturally the Krell qualifies, but you might also consider Bryston, Classe, Pass Labs, Bel Canto, and PS Audio. Tubes from ARC, BAT, or VAC might also do nicely. Again, just pick what sounds best to you.