I have the HD598 and I recently added both the Sundaras and the 6XX to my collection.
Sound is subjective, but to me the Sundaras sound more dynamic, and a bit more analytical than the HD598; however, the sound can be a bit fatiguing for extensive listening/playing sessions. Many people on r/headphones have suggested that they need to be EQ'd. I've heard they also need an amp to properly drive them, and I'd have to agree -- as I have to turn them up to nearly double the volume of my HD598 on my respective amps, unless I flip the high gain switch. Also, as firehawk12 suggested, I also don't think they'd do well connected to a controller.
I use the Sundaras with my PC connected through a Fiio e10k -- which is adequate to power them, but probably not the best. The e10k has a bass boost, which definitely helps with the Sundaras. I also use them with my Schiit stack to listen to vinyl, and watch tv.
Gaming wise, I've mostly only played Elden Ring lately (PC) with them on, and it sounds great. I'm not sure how they'd do for games where you need to pick out enemies around you in the environment -- though, I don't think the sound is that much narrower than the HD598. However, I haven't done any real A B comparisons to test this out.
On the music side, I think the Sundaras perform pretty well. If EQ'd, they have a solid bass hit in the lower registers. Keep in mind, I mostly listen to jazz, funk, electronic, and alternative rock.
One last thing, Hifiman aren't particularily well regarded for their quality control. I luckily didn't have any such issues. I bought my pair through Amazon, so I'd be able to easily return them if they didn't work out.
