What are the effects of up-sampling

My last CDP purchanse had an option to select non-upsampling or 24/96 upsampling. After listening to some of my new(er) high quality recordings I concluded that up-sampling had no benefits for me. The sound was more closed in, i.e. rolled highs, etc and I listening in non-up sampling. Recently I put on some old CD's, either early digital or re-master analogue, which erred on the bright/glary side of neutral, in upsampling made and they became much more listenable.

Is this a common experience with up-sampling? Is it perhaps just connected to my tuning of my system? Or is it just the way my CDP, a Raysonic 128, is configured? Or, am I just hearing things, or not?