@soix - I am not sure which part of what I wrote you disagree with. I own basically the same DAC as the OP (except the upgraded version) and I also own a pretty good (super low jitter output) DDC (link) that I have used with that DAC over an extended period. The USB input compared to the AES or S/PDIF inputs is apples to oranges. The USB input is asynchronous and performs mainly on the strength of the DAC's USB input. The AES and S/PDIF inputs are synchronous and rely more on the source clock and the quality of the signal provided. The main other factor is how good the streamer is and whether it is optimized for a high quality USB output. I have two very good streamers here now that are both optimized for their USB output. IMO, the DDC does nothing to improve the sound of the Merason DAC in my system. The USB input of the Merason DACs is highly optimized and well respected (better than most). Therefore, if the streamer being used outputs a quiet and well-clocked USB signal, my experience with virtually the same DAC is that, to my ears, the USB input sounds better than either of the others coming out of a well-respected DDC. That is the basis of my suggestion to put the money into upgrading the DAC. The Mearson DAC upgrades are well-regarded and reported to move the ball forward wrt sound quality by the owners who have had them performed.
All that said, even in my system, the sound between the two options is a pretty close thing and I could envision an owner who would prefer the sound through a DDC and the AES input. I find the USB input provides a little more body, musicality, and fluidity while the DDC into AES or S/PDIF inputs sound maybe a little more muscular. If I were on a budget, I would upgrade the DAC first and then I could always try different DDCs later.
You are correct that there are too many variables and that listening in one's own system is the only way to be sure of what a listener enjoys the most. Not everyone here can afford to try all the permutations and so they reach out for advice and suggestions. It is not unusual to get different suggestions from different posters. Just ask which USB cable sounds best ...