WVICK
Why not go in increments. You haven't specified which Krell you have, but why not go with best of breed. Certainly you will probably miss the control, slam, and overall ease and effortlessness of the Krell if you go to low powered tubes for amps, though you most certainly will make up for it in other areas, like naturalness, rounded more 3 dimensional images, etc. It's been my experience that with more (quality) tubes in your system you will experience a new dimension where you get less ss compression in the soundstage and start to hear more of what your WP7 I'm sure is capable of-rendering very believable images not only in a huge side to side stage but front to back with rounded, full form, and "action" as well as bloom and color. Be careful, since not all tubed equipment is created to the level of the Wilsons.
I have to say I stumbled upon a rather awesome combination, with Krell 750 monoblocks in my system last year, I decided to try out a tube preamp, the VTL 2.5, a rather inexpensive tubed preamp that I didn't expect much from. In fact, it completely and dramatically altered what I was hearing in my room with some very high end SS, and Dynaudio Temptations. In fact, many of the comments from above are helpful in relating what is possible once tubes enter your system.
If you go in steps, you can still benefit by keeping your amps, try a preamp (like ARC REf 2, VTL 7.5 or even 2.5, Hovland, Wyetech, and Atma-Sphere etc.) Obviously,the Krells are great amps and though sometimes not considered true high end are very true to what they are fed. Your WP while efficient have a rather rough impedance curve, so power does not hurt in that application.
Also be careful with certain preamps, like the ARC ref 2MkII which sounds almost like the Krell preamps and has a distinctly solid state signature, with some of the benefits of tubes. If you can keep SS devices out of your preamp, all the better. ARC recently introduced FET's to the input stage of that pre in its latest iteration, so again, tread carefully. Also, beware of very tubey sounding equipment as well. Some of the less well designed gear can tend to the overly warm, noisy, grainy, wooly euphonic, sweet (VTL 2.5, CJ,) while some of the newer more expensive pre's like CJ ART, Atmasphere, and the like have minimized much of the negative potential that tubes can bring to the table. The better tubed pre's sound clear, balanced and natural without being fat, slow and losing detail and speed.
Sorry to ramble. Feel free to email me if you want more specifics, as I've played around with Krell a lot and know where you're coming from. If you go with a tubed preamp and keep the Krell, stick with best of breed and have your cake and eat it too. If you want to go further, then take that step when you're ready.
