Just to have a reference, could it be possible to do some basic measurements of the 3.5mm audio output of the Raspberry itself? The latest driver integrated into Raspbian claims to improve the audio quality a lot (which is done in a pwm pure digital way) and be near cd quality. Maybe you will...
I was about to propose the same. Asus and standard c-media drivers are known to be buggy and implement filters that degrade sound quality.
In this respect, the author of the Uni Xonar Drivers also implemented a tweak on these drivers that disables an internal filter and resulted in subjectively...
I was looking for a cheap alternatives to the Speaka or Xtremepro USB DAC dongles (which are impossible to find at reasonable prices) and I found this...
On a similar price as the Topping offerings I think only the Melokin is available now. It was 300$ on pre-order but now it is 360$ plus USB. I found a recent review here, albeit on the subjective side.
Maybe if you contact them and show your technical background they may offer you one unit to...
May I suggest the Melokin DA9.1 DAC+head amp?
http://www.qlshifi.com/en/wzcapi/da9.1.htm
It uses the ES9038PRO DAC, instead of the stripped-down Q2M version used in the Topping products (DX7s, D50). It would be nice to see whether the full 9038 chip (which cost signficantly more and uses much...