Swapping between the two DACs, while listening to the same song, I just cannot hear a difference. So, I am wondering if both DACs just sound that similar or if I simply cannot hear the difference in them.
If you are after learning on how to develop and refine your skills in picking up and discriminate sonic signatures, please forget all that A/B blind listening test science bull s crap. All you will get out of this is to go nuts.
Instead, I suggest to not ! listening over and over to the same song(s) again.
Memory for specific sonic signatures is lasting I can tell you, even for decades. Its the same as with smells, where when coming across one you haven't smelled for a log time, you nevertheless "flash back" to that time you intensely have picked it up first time.
So what I do is to listen to a lot of music, preferably also to a lot I never even have heard. That's very easy to do nowadays by listening to many different web based radio stations or music videos. Well, of course I already have a selection of stations with very different signatures I already know their specific performance pretty well.
Key is, not to hurry through them, but relaxing and get the gist of the "house sound" of any certain station overlaid by the performance of the DAC and the rest of your setup.
You will find that there are quite some differences and its not any necessary to know the recordings played nor is MP3 quality of any hurdle here.
Already having a pile of impressions about differences in presentation by now, you may already miss THE presentation you are after, something sticking out here, something missing there, impressively exaggerating, not involving ... Or you possibly did come across something completely unexpected.
Now you can check back your impressions about presentation with recordings you know well. The net result is the sonic signature of the DAC under test (and the rest of your setup).
Do the same with the DAC you want to compare with in a second run any time later on and voi a la - there you are.
Have fun to explore the myriad of sonic signatures out there!
In case you can't pick up any major difference, so what? Enjoy, or have a look elsewhere in your setup if you feel the need.