Borrowed a HE-6 from a friend and compared to my HD650. He-6 has more details, better mid and treble. However, the upper treble is too high for most songs that I listened to. I listened to mostly current popular songs (pop, dance and soul genres). I have to use the EQ to lower down the upper treble. HE-6 is also lack of punchy and solid bass when compared to HD650. For the type of songs I am listening to, I still prefer HD650. As for comfort, HD650 feels more comfortable on the head. The pressure are more even on the head. I still haven't heard the LCD2/3 and HD800 yet. Until then, I am sticking with HD650.
By the way, HD650 sound awesome with Bifrost and Lyr and I still haven't upgrade the tubes, wires and fuse yet.
My setup: Laptop (flac) -> AudioQuest Forest USB cable -> Bifrost -> Lyr with stock 6BZ7 tubes -> HD650.
Sweet impressions. Even though the HE-6 sound is undeniably legendary, I prefer the HD650 over everything else I've listened to, that includes the HD800, the T1, and even the K1000.