David,
You should definitely check out the Audyssey thread (first post has the link to the FAQ).

Also, the CTO/Founder of Audyssey explained why the setting should be set to 120. In fact, he went so far as to say it shouldn't even be a tunable parameter (for LFE and LFE only)

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Denon 4520, EPIC80/500/VP180 Speakers