OP, I do second your consideration regarding a DAC.
I can imagine that the audio interface (which is primarily for home recording and/or dj-ing as far as I know) is, for some reason, the weak piece. IMO, it's not the amp nor the pc.
In order to test, you might want to invest a little money in a decent DAC (no need to spend more than 200 Dollars, really), using USB 2.0 and a media center software (if you don't have 1 alreaday) like jriver or foobar (no additional $$ investment for fb). That is what I would do if I were in your place.
Even without a change in the current lineup of all your gear, you should hear a difference in SQ. Although I do support the respective suggestions of other members of this community, which will further improve SQ.
Please comment on the results after you have changed whatever you intend to change. I'm sure all those who have contributed to this post will appreciate it.
Best, eagledriver