I can't speak to the integrated amp or to its power-section -- by all accounts it's quick and hits hard -- but I did have the two-channel preamp for awhile and I've played through most of the Shuttle and Shuttlemax heads.
The WJ definitely has a less attenuated/rolled-off deep low-end than any of the Shuttles. Probably smoother throughout and more pristine (as Jake says) and sweeter up top, too. Super-articulate. I seem to recall getting a little compression and just a bit of breakup if I dialed the Gain up high (there is a series of green, yellow, and red LEDs indicating your input level), but it's not a pre you'd get if you were looking for tube breakup, especially not in any traditional sense -- not gonna cop any old Jack Bruce tones with the pre alone. Clean, fast, direct being its wheelhouse, I'd agree with Jake on. Same with it feeling "big" -- bigger than the Shuttles.
There is an (optical?) compressor built-in. Didn't play with it much, but, as far as built-in compressors go, seemed nice to me (then, I am no expert on compression).
Depending on your setup, you might find an HPF beneficial with the pre. The amp might have a fixed-frequency one built-in (I believe that the cabs do -- I don't know that the preamp doesn't but, if it does, it's set quite low -- might be more of a subsonic filter than "HPF" -- don't take that to the bank, though!).