For headphones I am more concerned if you can run it off your motherboard. Some headphones require an amp and dac to run.
as for 400i vs shp9500 I would say the 9500 if you can afford the modifications due to issues of quality control with the hifiman company.
The modifications are fairly simple but they can rack up the cost. for the mic its simple… you want the V-moda Boom Pro microphone it just plugs into the headphone. For the pads you either need to follow an instructional video to cut and peel off the pads off the adapters without breaking the adapters or buy 3d printed adapters from Modhouse Audio(they are $10). For pads in particular, you want Velours for competitive and you can put a suede, hybrid, or leather for more casual ended use. I prefer suede as it keeps a good balance in sound. Dekoni’s pads work beautifully for fixing most of the clamp issue but it still fits rather loosely. For pad size you want the same pads that can fit onto a Beyerdynamic DT series(770, 880, and 990). Though for oval size I am not quite sure if you prefer ovals. You can of course use Brainwavz pads as they are cheaper but you will have noticeably cheaper materials for your pads and overall comfort is definitely very different.
Other alternatives if you want just a gaming headset. Grab the Sennheiser PC37x or if thats out of the question Astro A40 works well and is another open back typically comes with its own mixamp. Closed back would just still be MH751(752 if you want equalizer and 7.1).