I have a 400s, and bought a pair of 400i's figuring they had to be better than the 400s. The 400i is more detailed and really has some nice sounds to it, but the sound is small imho, compared to my 400s. The 400i's stage image seemed about the size of a grapefruit...small melon at best. The soundstage on the 400s is really open and has a live feel to it. Voices on the 400i's are great, but the S puts the singer large and right in your head. (try the album Linda Ronstadt- "What's New)...
The 400s may not be quite as "tight" as the 400I, but I LOVE the large open and lively sound of the S. My wife concurred.. she wouldn't even listened to the I's any more after a few weeks and we fought over the 400s all the time...I sent the 400i's back. There wasn't anything I preferred them over the 400s. I listen to everything ...as long as its recorded extremely well.
I listen to Tidal Lossless, I have a Creative Labs E5 DAC/AMP (which I LOVE), and a PA 1502A APPJ Tube amp. I have modded my grills and run Focus Pads A. I have never heard any other high-end headphone, but I am pretty familiar with what sounds good. For the money, I am VERY pleased with my setup.