I have read 4 articles and 3 said there's a hardware difference for the sound improvement that is only possible to get if you buy that model but also some software extras that the original should get via OTA. Let me see if I have the links handy...
http://www.mobileburn.com/22793/new...n-edition-for-sprint-hands-on-and-impressions
http://www.phonearena.com/news/HTC-One-M8-Harman-Kardon-Edition-hands-on_id55655
http://www.phonedog.com/2014/04/29/...rman-kardon-edition-partnership-with-spotify/
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I don't think you even read these articles, because none of them mention any hardware changes, upgrades or enhancements other than the in-ear headphones.