How important is a preamp for purely digital sources?

A preamplifier vs direct to amp to dac ,a good preamp always better especially a vacuum tube preamp it allowsbettrr dynamics 
soundstage depth as well as image depth .good power supplies 
and transformers give you the amplified dynamics ,
preamplifier it takes the signal and amplifies the signal.I have done many comparisons when I owned my audiostore ,as well as Audiophile consumer over my 40+ years in audio.

Enormously important!   However, there are not very many good preamplifiers on the market and the good one's are very expensive. 

Pinemoutain what do you base your comment on?

The answer is no. You seem to be fine as is , so dont drink the blue juice.

Check out a  Pre Box S2 Digital for a high performance good value DAC.