modmic doesn't work too well with my mixamp, it works but it's not as clear as it should be but it could be because i had it too close to my mouth. I've moved it further away now and had zero complaints since doing so but when i was looking online apparently they can have issues. but it sounds like the "streamer" mixamp the TR may be the better option for you if your doing all that.
HD598's is basically what I use (HD558's) it's the same drivers just slightly different grills and some foam tacked on. They are very easy to drive too easy IMO. I find I need to have them on louder than I would like (my hearing is very good) to get consistent power to both drivers.
So I have sound turned mostly all the way to voice chat to compensate for that then have to lower that slightly within the console. i think that is an issue more personal to me though my hearing is top notch so i tend to have things on a lot lower (speakers, tv, radio, etc) than others.
the HD595's I had before though had no such issues but that was on the older mixamp, it could be my 558's wiring isn't as good as it should be internally. however it's easily resolved by increasing the volume then dialing more to voice. hd598's are amazing gaming headphones (having owned 595's and 558's) no such thing as a proper upgrade from them tbh, i wish I had kept my 595's they were that good.