oversample the digital input anyway. Does it therefore matter if the input is upsampled or not? Unless there is a super-duper interpolation algorithm that can create more information from that which is encoded in the 1fs file, I don't see how upsampling can ever improve the sound, over and above what oversampling will do anyway. I've tried many upsampling schemes, and like you Romy, I've reverted back to playing all files at their native rates. Upsampling seems to kill the sound, though interestingly, oversampling doesn't (at least on the PM Model Two). Of course, this is very much dependent on the quality of the filters used.
FWIW, my feeling is that oversampling is absolutely necessary for rates below 4fs - I don't agree with NOS DACs for 1fs. At 4fs and above, I don't think oversampling is necessary and that NOS may work well here, though of course all delta-sigma DACs need to oversample at 128x or 256x in order to work anyway.
I'm no expert, but my understanding is this. 99.9% of DACs are going to