I finally got around to plugging my HD650s into my relatively new Schiit Modi Multibit and Magni 3 combo and the result was nothing short of WOW. It sounds as though the bass has been put on a course of steroids, it's more powerful, digs deeper and has more impact, it really does sound good. The imaging is slightly better than with the original modi and Magni 2 but still falls well short of the HE400i, it is still mostly, left blob, middle blob and right blob. The actual timbre of the sound though is wonderful though, very natural, never "shouty" or harsh, just layer after layer of smoothness at all frequencies. One area where they do fall short of the HE400i is comfort surprisingly, I find that my ears get heated up considerably faster than with the HiFiMans, probably due to the extra room afforded by the round pads rather than the quite snug fit of the oval Sennheisers.
So which is better, HD650 or HE400i? I honestly can't choose, they are both exceptional but with different strengths and weaknesses: The HD650 has more bass quantity but the HE400i has better bass quality and speed, The HE400i has very smooth, natural mids but the HD650 trumps it with some of the best mids in the business, and on and on it goes. I'm just very glad I have both