So, I exchanged my HE-400 for the HE-500. I must say I was very pleased within the first 30 seconds of a song through the HE-500. To my ears, the HE-500 has a fuller, cohesive sound that suits my tastes. Interestingly I notice more bass impact with the HE-500 although the HE-400 seemed more controlled. The HE-400 definitely is a good headphone, but for me, the cost difference is well worth the upgrade.
As an aside, I publicly would like to thank Justin at Headamp for top notch service and incredibly fast shipping. Without a doubt, this is the guy to buy from!