On a multi layer related note - I picked up a pair of Hifiman RE-262 IEMs in trade today. I'm quite impressed with them. I think they are similar to my HD600 in that they are very good, but still have room to go. The tonal balance is good, but lacks clarity in the upper end (also similar to HD600). The 3D headstage is quite present and superior again to any full size headphone I've ever worn. I really see this as my new direction. The HE-6 just isn't doing it for me.
Unfortunately even with a fairly high impedance (150ohm) and low sensitivity (95db) I still get substantial hum from my MKVI so the best amp I have for these presently is the built in amp from my Shanling CDP, which doesn't sound amazing, and couple that with the subpar DAC it also has inside and I really can't say for sure where these IEMs will go. Listening through the hum on the MKVI shows much better treble clarity, but the hum is too distracting for detailed analysis.
Still on the fence about trying the speaker amp route with the HE-6. They still sound best when flipped around and cables swapped. The only reason I haven't done something to get rid of them yet is if more power makes this flipped orientation more lively (more forward, mostly) it could still serve a purpose.