I have the pb2. It a good amp , I use it with my hibino hdpr10. It improves the sound big time even with the jh16 iems
Comparison thread in my signature for the three I've tried.
What follows has to be read with a fat 'In my opinion' before each sentence. Also, I don't have 'golden ears' though my hearing range still extends to 17 kHz at age 41. And I listen almost exclusively to classical, also an important consideration.
I own the HE-500 and HE-6, have briefly listened to the 400 but not enough by far to 'know' them.
Very briefly about the 500:
If I'd have to describe the sound then I'd say 'on the warmer/earthly side'. The HE-400 have a V-shaped profile with emphasis on bass and treble and a recessed midrange from what I've heard during the brief audition I gave them. The 500 advances the midrange a tad. It is absolutely glorious with vocals and string instruments. Not bass shy at all though. The treble is good but due to its warmer voicing not as pronounced as with the HE-6. Soundstage is good but it's not a HD-800 and not as large as the HE-6. Their 'weakness' lies in a bit of lacking transparency and ultimate clarity but that's also what makes them more forgiving of imperfect source material. They're easier to amp than the 6 but still you'll need quite a powerful amp. EG: they're glorious on my Violectric but amps like the Fiio F17/E09K combo don't drive them well, the ODAC and its amp are marginal: it works good but there's still an important gap between its performance and the Vio.
Very neutral all across the range, works with all musical genres. More transparent, detailed, lively vs the 500 all the positive adjectives come into play, the treble can be -on some occasions- a bit too much. Not as analytical as the HD-800, which imo is in most cases a good thing. Only one disadvantage and you need to consider this very seriously: they're extremely inefficient and you imperatively need a serious amp to get the best out of them. Hifiman recommends at least 2 W/channel but I'd say that's too low. Compulsory reading: the thread about 'amps that can power the HE-6 well'. It's long but trust me, you'll want to read it. I power mine at the moment with a Violectric V200 (2.75 W into 50 Ohm). To my ears it powers them well and on the thread I mentioned the consensus is that the V-200 is among the few desktop amps that have enough oomph. That being said, a number of people out there who are no fools are adamant that in order to drive them to the fullest, you really need to run them from a speaker amp. I'm reserving judgment but have ordered a good speaker amp to see for myself.
My advice: if you're considering a 6 you have to be prepared to invest money, time and effort to get the best out of them and it makes sense to build your system around them, carefully picking components. For a good 'second best' solution if you listen to classical/vocal/jazz then having to pick between the 400 and 500 I'd go with the 500 for sure. The 400 I can see as being good with electronics, metal etc but avoid it for vocals and for sure classical music. Can't say anything about the other cans in the Hifiman lineup.
Comfort is the same as with all other HiFiMan cans except perhaps the 300 due to it being a totally different design that weighs less: basic and utilitarian. These are not light headphones. Some people complain about wearing comfort during extended auditions, I listen about 8 hours/day and only occasionally it becomes too much in terms of pressure on my cheekbones (but I have an angular face and those are pressure points in my case).
I think the HE-4 is more power hungry than HE-500. From what I read , the HE-4 needs something like a minimum of 6/700Mw @ 38 OHms to provide full dynamic range for a listening volume of 90db ( I'm not sure for this last value.. it can be 85 maybe ) . The M-stage seems to fall a bit short if we consider specs.
HE-4 would have a sensitivity of 86 dB, HE-500 is according to me about 90. Innerfidelity has it at 89. So the 4 is def. significantly more power hungry(it's a log. scale) than the 500. To what extent this means something for this particular amp I don't know as I'm not familiar with it but judging by the number, things don't look to hot: it pushes 400 mW it 60 Ohm so even being very generous that would mean say 800 into the HE-4's 38 Ohm--> looks insufficient to me to drive it fully (you'll get some sound but volume might be very low or it might run out of steam). Once again: I haven't heard it.
... I own the HE-400, but sorry to say, I seldom use it... don't get me wrong, there is nothing wrong with the HE-400, it is a really nice can, especially if you consider the value for money relation.
The problem here is, that there are some other eve better cans that use up my limited listening time: 1st the Audeze LCD-X, 2nd the AKG-1000 and 3rd the Denon AH-D7100.
All of them play in a higher price-range and in my opinion also in a significantly higher sound-quality-level. Some might question this for the Denon due to it's rather pronounced low frequency range. But I really love it! So the HE-400 has to share the little time left with the AKG K-702...
But still, there is another reason why I don't utilize the HE-400 more frequently: It is the pads. Here I really would like to see something more comfortable. I am actually experimenting with some alternatives to their original pads. If successful I will update here.