Weird thing is that even though they have deeper bass than the HD650, they sound bass lite comparatively. They require a sizable bass boost. The upside is that they can handle it without falling apart.
Frequency response isn’t everything and though I can’t support it with evidence, it’s obvious to anyone who has much experience in audio. We shouldn’t be afraid to admit it as objective-minded humans.
As you can see bass extension is very similar. Also note that HD650 is often said to lack low bass.
From 70Hz to 200Hz the HD650 actually has 2 dB more bass and is why the HE400SE is relatively a bit bass shy.
Notice how the slope (tilt) from the HD650 from 100Hz to 1kHz is downwards and from the HE400SE is upwards ?
This means HD650 is a bit warmer in the mids.
The small bump between 600Hz and 1kHz gives a bit more forwardness/open-ness to the HE400SE as well.
Its not by much but this makes the HE400SE a bit more dynamic sounding.
Now we come to the elephant in the room.
From 300Hz to 4kHz we see the HD650 is closer to the horizontal line (which is audible 'flat' in this plot) where the HE400SE, in good accordance with all other Hifiman models, dips. In other words the clarity in the HE400SE is definitely less. This is heard in voices the most and in instruments like electric guitars for instance.
You can EQ this back in. I would say that for listening to classical and well recorded jazz a little extra presence is needed.
For pop and rock music as well as for many older recordings the dip helps but when this is not too much it will take the 'edge/hardness' down.
Above 6kHz there is another substantial difference between these headphones. From 7kHz to 15kHz the HE400SE has a very audible 6dB lift opposite the HD650. This is above the sibilance area (5-7kHz) where the HD650 peaks a tiny bit but not enough to become sibilant.
From 7kHz to 12kHz we enter the realm of sharpness. It is very easy to see that even though there is a 'dip' in clarity there is a sizable lift in sharpness in the HE400SE. So cymbals, violins and other 'nuances' from instruments with harmonics all the way up there are accentuated.
For classical music and especially people above say 45 y.o. this works really good and adds some 'air'.
With some recordings and some people being sensitive in that area (alas for me) the sharpness is too much and especially with trumpets in jazz this results in some sharpness.
Then we haven't even discussed the slight angle difference of the drivers and different driver-ear difference which gives the HE400SE the edge when it comes to 'stereo imaging' and 'soundstage'. Despite the dip in clarity (which doesn't help with stereo imaging) the angling of the driver is of importance.
The above is actually obvious to anyone who has much experience in audio.
And indeed we shouldn’t be afraid to admit it as objective-minded humans that frequency response isn’t everything but is at least it clearly is in your assesment (which isn't wrong) and this can be supported it with evidence (Harman and other research) and as I did above.
So yes they are different. Just like any other headphone differs. Yes, preference exists. Also new toy syndrome exists.
No doubt the HE400SE is a very good entry in the planar world. I own both HD650 and HE400SE as well. Both can do with a bit of EQ.
Out of the box the HE400SE impresses more. Enjoy but keep your HD650 around or EQ.