I think it would be unusual for someone not to hear a very noticeable difference with and w/o a DAC and you've already stated that you hear a difference. Part of the problem may be that you also must need to sit back and enjoy the music. I have made changes where I couldn't tell a difference at all (the largest SQ change for me was a DAC) but later when I wasn't thinking about anything I thought to myself -oh I hadn't noticed that before or wow, that sounds good - I then realized that I did hear a change. But the whole A/B thing and the analyzing, for me anyway, makes it hard to notice differences.
But yes, you also need to make sure you don't have any weak link in the chain anywhere. Make sure you use USB 2.0 certified cables, make sure all the volumns are up (even in windows), make sure the DAC configuration page is set to the MAX bit rate (I had to change mine, I'm not entirely sure it effects the DAC but I got to the selection after right clicking on the driver for my external DAC), use good software etc, turn off all sound processing, etc,
The device may even sound better after you've been listening to it for a while. I have never listen to any uDAC but I think the e7 (my cheapest DAC) sounds great and I'm guessing uDAC is similar(??).
I had some other thoughts too but...