I'm happy driving my W/P 6's with Pass X600 monoblocks. The Pass's have pure, smooth, liquid midrange and treble ranges and are OK if not great in the bass. This is a good fit with the W/P 6 which tends to be a little hot and very revealing in the midrange and treble. I don't think they need a lot of help in the bass, so the Pass's are fine. The X600s put out more power than I need for the relatively efficient W/P 6's, so maybe a smaller Pass would be more appropriate. I listened to the Levinsons and thought they were a little veiled or laid back. The Krells sounded a little colored, especially in the bass regions, amazingly for a product at this price level.
Levinson or Krell
I am currently putting a seperate listening room in my home and have listen to a wide variety of equipment. I have a pair of Wilson Watt Puppy 6's and I am currently shopping for amplification. I also own a Mark Levinson 38s pre and a Wadia 270 transport\Wadia 27ix dac. I have heard the Krell 350mcx's on the Wilsons and like the sound very much however I have heard lots of good things about Levinson 33H's paired with these speakers. I have yet to hear the Wilson/Levinson combo and I am open for suggestion.