Not sure whether this has been spelled out yet: I'm assuming the mp3s are 16 bit (128k for 24 bit would be pretty unusual) but (at least) the FLACs are 24 bit. Downsampling is a lossy process (and the quality of the down sampler matters) so I didn't bother and have instead generated 24 bit mp3s. I believe the down sampler in dbpoweramp is well regarded.