What color do tube DACs add? Green, Blue, Chartreuse?
Matching the inputs to what your source does best will allow you to realize the full potential of the DAC's presentation. After that it's matching a DACs sound presentation to your system to create your preferred listening environment.
Whether you use no tubes, all tubes or a mixture to create it, the environment you prefer is the only important factor. You must first decide what you are trying to achieve and then build the "system" to get you there (realizing that along the way your priorities will probably evolve.)
Changing the DACs sound can be as easy as using a different input or connecting cable. The source matters and contributes to the presentation and should be the foundation on which you build a digital system.