I also have a Topping D30 for my desktop. When I was using it with the Crack + Speedball (HD 6xx) I noticed that the combination had some harsh highs for my ears.
However, after switching to different sources and listening to the same songs, I noticed that the harshness went away. I had some problems with my speedball, so i had to take it out for a while.
The Topping D30 used with the crack and without the speedball had much more manageable highs for my ears. However, the speedball matches so well with my other equipment, so it would be disappointing to not have it in.
Everyone's ears are different, so what I like may not be for you, and the other way around.
A side note for the D30, sometimes mine stops producing audio form time to time. Power stays on, but nothing is coming out. This seems to be documented by many people.
So while it may be a good cheap DAC, there are some known problems out there.