imo MP3/ AAC wouldn't be used for anything higher than 320Kbps (maybe 350 ish, (that's what ATRAC goes upto)) you'd go to lossless from there up, and depending on your library size and available storage just use ALAC and be done with it
I guess my ears have been damaged with years of being around loud music because I cant really tell a difference between 256 and AAC and I have a pretty good setup with martin logan's, adcom amps, onkyo receiver, and audioengine DAC.
I suppose if someone with a better ear were to sit with me and point out the differences, I would know if im hearing them. I did notice with the DAC, the sound did seem to get more depth, but I also accomplished a similar effect by just a slight tilt change on the martin logans.
there are times when in certain recordings, I hear things that I normally dont in a car, or from a CD. but lots of times I chalk it up to a placebo....
I bought a pair of B&W P5's last week, took them home expecting to be wow'ed, and I wasnt. they were nice, but not 300 dollars nice. I expected to have my socks blown off with some of the reviews I had read.