In blind tests, most people can't hear the difference between a CD and 256kbps MP3, and the difference virtually disappears for everyone at 320kbps. I'm sure some people will disagree, as this is one of these "amps sound different, cables sound different" kind of topics, but that's what I've read and experienced for myself.
The first three Zeppelin albums have been remastered. And I read that Jimmy Page wants to remaster the whole catalog. You might want to check if your download is remastered.