Are you guys on crack or do you just not listen with your ears?
Nobody here has talked about how Zep or Andy Johns or whoever, used compression.
Why shoot, I stumbled onto that about 30 years ago when mixing a 4-track tape with drums on one track. By accident I left my little guitar effects COMPRESSOR on the track ... boom, instant LEVEE BREAKS drum sound.
So I am pretty sure that in addition to the famous staircase and the famous 3-microphone drum miking ... that the other VERY KEY ingredient was pretty heavy use of compressor. Probably tube compressor at the time.
Anyway ... there is more to Zep sound than just that. They seem to have (like the Beatles) rolled off a lot of top end in their mixes. Doesn't anybody else notice these things or pay actual attention to the actual records? Sheesh