wilson watt puppy 7 and krell 400cx...which preamp

Agree with the tube preamp suggestion. VTL and ARC also go well with Krell. I personally would go VTL though I've not heard the BAT. You'll get more 3D imaging, and less 2D ultra detailed presentations. I'd have to say with any of the tubed pre's you suggest, you'll have a world class system you can put up against ANYTHING.

Audio Research Ref II mk II!
This is a killer setup, I've heard it before!

would defintiely go with tubed preamp.

ARC, VTL, BAT, etc.

CAT also goes well in that system...

If I may I would like to suggest the Audio Note M6 tubed preamp this is without a doubt one of the best preamps on the market. The good news is that since they are not that well know you can pick them up used for an excellent price. The m6 list for a little over $14,000 new and can be had used for about $7,000 They are an absolute must hear if you are serious about a tubed pre amp solid state amp pairing. I would also consider the Pass x600 mono amps these amps have some of the best dynamics on the market over 153db and mate well with tubed preamps, considering how excellent the Wilson speakers dynamic capabilities are I would think this is a match made in heaven not to mention the Pass pair can also be purchased used for about $7,000 to $8,000 used on Audiogon. 600 mono watts versus 400 stereo watts easy choice I would think, not to mention better dynamic capabilites.
My final concern if I were you would be impedance matching make sure the tubed preamp you choose dosen't have to high of an output impedance a trait common with tubed preamps Krell stipulates in there manual that if you want to use tubed preamps with there amplifiers you must first have the output buffers adjusted by Krell to handle the higher impedance output from a tubed preamp if you don't send them back to Krell for the adjustment you void your warranty ! definitly food for thought I would think. Good luck sounds like your well on your way to a great sounding system!

Definitive

The statement from Definitive is not correct. First off the internal adjustment does not have to be done by Krell, any dealer can perform it as well as yourself with out voiding the warranty. Its a simple flick of a switch on the output stage after removal of the cover. No big deal. Also with quality tube designs as Audio Research there is no need to perform this task. The modification is done to prevent damage due to DC leakage from inferior tube designs. I ran my Ref II mk I into my Krell FPB-300 without any problems.