Otoohot:
Tough comparison. Personally, I don't believe the Krells are in the same league as the Pass 600s. The X600s are Pass' best sounding amps and absolutely light up the cheaper Krells or Levinsons. Depending upon which Krell you're considering, the Krell may stay close on base, but everything after that falls far behind.
Until recently, I used N802s with my X600s and the added performance I got out of my N802s over my previous amps was incredible. I consequently did away with subwoofers due to the added base extension and also gained an incredibly low noise floor and amazing dynamics, not to mention an "airier" top end. The N802s tend toward the bright and harsh side, but the 600s seemed to tame this without rolling off the highs (like the Krells, Proceeds, or Levinsons). You'll also find the Pass gear to be much more accurate.
I've since upgraded my N802s to Eidelons, but I stand behind the extraordinary experience with the N802s & X600s given the price point of the N802s and capability of the N802s to continually rise to the level of their upstream components. If you like your 802s, you'll be in love powering them with the 600s.
Good Luck.