For me.. via RME ADI 2 DAC
Hifiman He400se > HD650 (too dark (haze effect) with lower quality bass) = HD560S (The most dynamic of the 3, but a bit sharp in the treble, with tight bass)
Hifiman He400 is => neutrality with faithful bass, wider soundstage, you can wear it all day, it's just perfect to my ears (no EQ needed)
PS: The 2 Sennheisers are tiring (a little) compared to the Hifiman
Only defect, it is more difficult to drive than the other 2 Sennheisers
Easy to feed:
HD560S > HD650 > Hifiman He400se
They are all 3 excellent headphones but if I had to keep one headphone for life, it's the Hifiman He400se by far.. be careful to go your way if you don't have a beefy headphone amp
For me that i have all Sennheiser HD6 line , some Hifimans and HE400se , i would say that i love 400se for the 100 euro are amazing. As u can see in the photo i use them lot.
But HD600 has better neutrality better clarity
HD650 has smoother warmer sound
HD660s have better imaging better texture and separation tighter bass and soundstage that is close to HE400se.
HE 400SE on the other hand have low end that goes deeper but metallic not organic sound is there compare to 6 line.
Honestly 560s is closer to technical level of 400se in my ears . And for me HE400se sound better than 560s.
About comfort HD6 line is way lighter on the head doesnt move on the head , the problem is clamping force that goes away like everyone knows, it doesnt have the Hot spot on center of the headband.
About power like you said these need a lot of power more than HD600 to sound amazing... And that makes them None adorable for people getting on the hobby, its mostly for people that already have good dac amp... So they lost value fore money aspect... And i am not even saying about cable replacement.
About if u have to keep one for life ... Trust me Hifiman and Keep for life doesnt go together . And thats not the case with HD6 line that those yeah u can keep them for life.
I understand the hype and how amazing these are for the price i know i use them every day , but in my opinion they cant touch HD6 line. On the other hand Sundara and XS yeah they have something special even if they also sound metalic.