I don't know how the Wadia pre works, but there are only two ways which it can work:
- as a digital pre. This is definitely inferior to an analog pre, because it throws bits away, compresses dynamic range, and brings the signal closer to the noise floor. I have heard the effect of digital pre's and let me tell you it is not pretty.
- as an analog pre. The DAC outputs its full signal into a conventional analog preamp. The question then arises - if you are going to run the digital signal through an analog preamp, then why bother using the Wadia as an analog pre? You lose the advantages and flexibility of having a seperate pre. Why not just take the signal from the DAC and use a seperate pre?
People who might argue that the one box solution is "simpler" may as well argue that integrated amps are better than pre-power amps. A DAC with volume control integrates the analog pre to the DAC, in the same way that an integrated amplifier integrates the pre to the power amp.