Changing the DAC frequency from track to track is a bit of a non-starter, because then you can't do things like cross-fade. So you have to pick one and stick with it -- and the overwhelming majority of MP3s are 44.1KHz. All sample rates other than 44.1KHz are resampled in software in all car-player versions. (I have a funny feeling that one of the Rio portables really did have a hardware-fixed DAC frequency, and actually resampled everything to 48KHz, but that might have been one of the ones that was never released.)

Peter