I own a Wadia S7i, and I have ekspermitteret with preamps with my S7i.
The conclusion was very clear. There is a HUGE difference between preamps, and there is also no doubt about, that a good preamp does it _much_ better than the digital preamp built into the Wadia S7i. For those who may doubt my conclusion, try using a Classé Omega MK III, Tidel Preos or Viola Cadenza.
The cost for a good preamp is definitely worth the price, compared to the improvement of the sound.