Sorry, but something appears to be wrong with the description of your setup. You describe it as if your current RME device was the clock master and would influence the A/D conversion of your UA Preamp.

When I look at the devices it is not possible that the current Multiface or even the planned Digiface USB could become clock master and thus have an influence on the A/D conversion of your UA preamp.

I rather believe that your UA is the clock master in your current setup and nothing would change if you would replace it with a Digiface USB.

It starts with the point that you UA preamp doesn't seem to have a single digital input port to be able to receive a clock signal from the RME device. In such a case you would need a RME recording interface with Word Clock support, as WC is then the only solution to get the clock for the UA.

If you look closer to the UA's digital ports:
- the AES port (DB25 plug) seems to provide only 8 AES output ports but no inputs and
- there is no ADAT IN port

Please check your setup again and clarify this.
If you would like to have an RME device as clock master, then you need to get a RME recording interface with Word Clock.

Whats better for the UAs conversion quality
- to get clock via Word Clock or
- to be clock master and send clock to the RME device
I can't tell you as this depends on the implementation of WC in the preamp.