Rock recordings of that era are notoriously compressed.While I like Led Zeppelin and Aerosmith very much,both bands put out some of the worst recordings ever,especially when transferred to cd.You might want to pick up a Japanese engineered re-release,they usually do a decent job.Check out cdjapan at the link below.Be aware,if you order from them,it will take approximatly a month to recieve your package.Good luck.
http://www.cdjapan.co.jp/
http://www.cdjapan.co.jp/