Based upon internal images of the Tubedac11, there is a single XO crystal oscillator for the TE7022 USB input, but no other XOs to be found anywhere else in the circuit. Thus, I can only assume that any clocks are of the PLL (phase locked loop) variety.
That being the case, you definitely want to feed the DAC the highest possible quality SPDIF signal, as there is no high quality onboard reclocking within the DAC itself.