I don't know why but I can hear differences between...
How do you go about listening for these differences? Is it through a controlled listening process, or you just notice differences after you hook up one or another?
If no controls are put in place, what you hear will not mean as much to you as you think, and it will mean even less to anyone else...
Set up a level matched (+/- .1db) double blind test with nearly instantaneous switching, and there are many who would bet embarrassing amounts those differences would disappear...assuming we aren't dealing with headphones and impedance/power issues. That definitely adds another element, but that's not magic either, just choose a device that's competent for the job.
Do you use different filters? I myself haven't managed to hear filter differences...my hearing has likely dropped below where that threshold could matter, but for others I have no doubt they could be audible.
Not trying to give you grief...just one possible answer as to how they could sound different to you...