Got my Planar yesterday, def need some time with it but first impressions are 1.This thing is massive. It's comfortable but heavy for sure and looks massive on my head. 2. The sound is excellent and it's well tuned, compared with my Sundara I find it to be very similar to the Sundara after EQ. In terms of detail, clarity and resolution the two are neck and neck, the Sundara needs EQ though where this is tuned better out of the box (Although still benefits from a bass to sub-bass EQ Shelf). Soundstage seems to be a hair wider on the Sundara and comfort goes to the Sundara by far for longer use (After 30 min with the Planar my neck was feeling tired)
Initial impressions vs. the S4x, the S4x is extremely intimate, it feels like a closed back headphone to me and putting your hands over the ear cups does not change the sound at all compared with the other two. Details are very good on the S4x and stock tuning is nice, you can def hear the treble dips, but this makes it very relaxing and easy to listen to. Oh, and this is probably one of the easiest to drive headphones I've ever used. My Tinhifi T2 Plus IEM's are easier to drive than these.
I'm using a Fiio K5 Pro to drive all 3 of these and a mix of FLAK files and Amazon Music desktop app. Tested a wide variety of music types but def need a lot more time with these before deciding. With that said, i'm pretty sure I won't end up keeping the planar due to its size and weight as I wear my headphones for 5-8 hours a day.
Edit: One thing I forgot to add, in terms of drivability, these 3 are almost perfect for the K5 Pro. The gain switch has 3 positions on the K5 pro, the lowest gain switch works perfect for the S4x, the Middle switch works perfect for the Planar and the top gain switch is perfect for the Sundara. I can keep it at the 12 O'Clock position and just switch between gain settings and it's nearly perfect.