I had the Denon D2000's and the HE-400's out of the box beat the D2k's.. although, The D2k's are great in their own right. So, I wouldn't say a "night and day" difference, rather a "dawn to afternoon" difference, but the difference was still there. I fell in love with HE-400's gorgeous mids.
Curious about that HE-500 now... I'm spoiled on good sound. I want the next best thing now.
Be carfule you might fall down the never ending rabbit hole. I know I did..