I own a set of HE-400S and they are smoother than the 400i. I had a chance to compare them and the 400S is the way to go if you are buying the HiFI Man HE-400 cans. The weakness of the 400S is in the bass. You can get a set of Focus or Focus A ear pads from HiFI Man and that helps on the low end. The addition of the Focus pads plus some equalization and the HE-400S is a beautiful listen.
The rest of the spectrum on the HE-400S is completely smooth and a joy to listen to. There is s slight hint in upper frequency grain if you listen very carefully but the HE-400S cans are much better than their asking price would indicate.
The HE-400S are light and easy to wear over long periods of time. It is very easy to enjoy their sound and a lot of people love these cans. The Inner Fidelity web site rated the HE-400S very high and prefered them over Sennheiser HD-600 or 650s due to their smooth and forward sound.
The HE-400S is also very easy to drive but I recommend that you use a good quality headphone amp for the best possible sound quality. Planar magnetic headphones can be power hungry but the HE-400S are pretty efficient. Here is the full review of the HE-400S as performed on Inner Fidelity: Inner FIdelity HE-400S review
Ed
PS. I don’t use my HE-400S any longer because I bought a set of Focal Clears. They are still a great set of headphones and for $300 you will have a really hard time finding anything better.