Hmmm, I have no experience with DAC's as my gear is used in a portable setup with my D2/Clip as source. From what I understand DAC's only make a subtle improvement in sound. But again, I have not used one, so my opinion is based on hearsay. If I had to recommend one that you listed, I would go with the ibasso, I have read good things about it.
Hopefully someone with more experience will chime in on the issue...
Anyway, enjoy your SRH840's great sounding cans!