I had posted a different reply that you may or may not have read. Either way - thought it was too snarky so I deleted it. Second attempt…
What I hope though is that it told you foremost that you have an awful lot of investigation work ahead of you to rule out the possibility that your observation was caused by one or more factors other than the upsampling difference.
Of course.I did try upsampling the same 44.1 PCM file to both 768k and DSD256 for comparison. Based on my own hearing, 768k sounds a bit harsh when compared to DSD256.
For me, I mainly use my headphone for comparison (as I think I can hear the difference more clearly). I can put on my headphone and listening to the the same album over and over again with DSD256 upsampling for more than 12 hours but I cannot do the same with 768k. I did feel fatigue with 768k around an hour or two. IMO, this experience do tell me something.
What I hope though is that it told you foremost that you have an awful lot of investigation work ahead of you to rule out the possibility that your observation was caused by one or more factors other than the upsampling difference.