Can I ask the question that shouldn't/mustn't/oughtn't be asked in these circumstances.
When you were listening, did you know what type of filter was being used or did someone else set the filters and you listened out for differences ?
I know this is the wrong question to ask here but would like to know the test conditions as they were.
My question is not intended dismiss what differences you (and many others) heard nor to invalidate all the hard labour and many hours of intense listening in any way nor to discredit all the work put into it.
It is more to satisfy my own curiosity as I am one of the persons with a reasonable hearing but cannot hear differences between DAC's and 'properly designed' amps. One might call me cloth eared, biased or deaf but recognise the idea that some may have better hearing than I.
I DO know that there are lots of audible differences between down/up-sampling algorithms but this is because they were not properly designed tested.
This doesn'thave anything to do with the ringing in filters which only happens with steps and not with sinewaves and steps do not occur in music signals.
Bad algorithms show all kinds of aliasing products bouncing back into the audible range.
Some even are barely attenuated in some cases.
There seem to be somewhat more well designed than poorly designed algorithms by the way.
See for yourself:
http://src.infinitewave.ca/ (set to sweep)
The part to the left and right of the yellow line should be pitch black (maybe dark blue is accepatble) but bright lines, especially on the left of the bright curve are a big nono.
These are ONLY of downsampled samples going from 96 to 44.1 in which case 96 is not a multiple of 44.1.
I am willing to bet that 96 to 48 would have given better results for most resamplers though.
When someone for instance has used a bad downsampling algorithm and compares a downsampled file with the original one they might easily find the downsampled one sounds worse yet done with a good algorithm may find there are no audible effects (when tested blind)