No, the X-10D is not a DAC. It's just a tube buffer stage. You could just attach it after the DAC and before the preamp. However, there usually is no need for such device unless you are using no preamp or a passive one. Some poorly designed CDP with low outputs might also benefit.
Musical Fidelity X-10D
At present I'm running a pair of Pioneer DV-727Fs into an SAE 2100L pre-amp via a Musical Fidelity X-10D. I'm considering the insertion of an MSB DAC III, with the upsampling upgrade, into the mix. Would I still benefit by keeping the X-10D, or will the DAC III make it redundant?