Any A900x user here? Do I need a soundcard or headphone amp to drive this cans? Mainly intend to use it with PC.

New owner of ATH-A900X here.

So many ATH-A900X. I freaking love this headphone after nearly 80 hours on it.

Agreed! It's perfect for vocal fans and instrumental musics. I'm a new age music listener. Paired it up with UD100 DAC by chchyong89.
The bass is deep enough for me. Muche better than other open back headphones I've ever tried. Instrument sounds are clearly defined, so much clarity. Vocal sounds so lively esp on high notes and does not hurt your ears. Mids more pronounced. Just Wow!
Soundstage is huge, good for positional audio, boomier bass in games as a direct comparison with Razer Chimaera.
Ear pads are not made of that common shiny pleather. Maybe it can last longer with this kind of material? Even I've got a big head, it still touches my jaw line just to say how huge the headphone is. It does not score full marks in comfort department however it still is the most comfortable headphone/headset I've ever wear. Maybe tying up the 3D wings together with a rubber band will solve the problem? Uncomfortable pressure still can be felt on the side of my head after wearing it for long hours. Isolation is good enough - hardly can hear outside noise when on music, music off still not so isolated well.