I guess receiver makers don't like advertising "what was wrong" so release notes are hard to come by. Over on AVS, there was a guy (captavs) who maintained a FW tracking thread. You might also try searching for "denon batpig" to see if he has a release note.

Finally, European site (so I've heard) may have release notes.

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Denon 4520, EPIC80/500/VP180 Speakers