One thing that is prominent to me on that album with Plant's vocals is that he uses a very short delay on them. It almost sounds like what you would use on a rockabilly guitar track, but not even that long. It just sorta fattens things up. I think that is the main thing you are hearing. There is some standard studio reverb in the mix as well but mostly it is a few milliseconds of delay (set for only one repeat).
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