Can I connect multiple sources to an external DAC

Exactly, sound improvement and normalization if I can call it that. I'd like both sources to have the same quality. From all the comments, it seems like there are many options for external DACs that can switch between sources. 

Correct -- both the Topping and Schiit examples I gave have a button on the front panel to switch between inputs.  There are some DACs that "autoswitch" based on which input is providing a signal. In your case, that could be confusing to the DAC if both sources were playing at the same time.

  I think it's all been covered, but it doesn't matter that the DAC gas only one output to the preamp.  You can connect multiple digital sources to a DAC, but only play one of them at a time