I bought a half decent xonar sound card for my dt770s but the reality was that my asus mobo had passable on board audio so that upgrade didn't yield much at all.
The biggest difference came when using a headphone amp, just the Fiio e09k made such a difference, I've been happy ever since. It seems that the DAC part most half decent sound cards seem ok to me, you need a decent amp to drive the headphones cleanly Imo.
However, I do have the upgrade itch and am thinking of getting the kit I listed. The Modi is a half decent DAC, the Magni 2 uber is a half decent amp and the DT1770 are getting good reviews.
I certainly wouldn't spend more than £250 on the DAC and amp, you are well into diminishing returns by that point.
And to multi quote hit the multi + button on the other posts you want to quote, then hit quote on the last one.
On the Little Dot, my ears didn't like it, I think I like very clean bass and impact, possibly because my ears are a bit shot form working in nightclubs and riding motorbikes, but as with coffee, everyone likes something different!