Thank you for your in-depth response! I will be using solid-state amplification.
Can you chime in on a comparison of comfort?
Also, I'll mostly be listening to Videogame OST and Chamber music.
Is there one headphone that would be a better fit for these styles of music?
I have heard that the K812 has better depth in soundstage while the HD800 has more height and width. Is this your finding as well?
I think the K812 feels a bit less on the head/face, but both are very comfortable to me.
Slight edge to K812 unless you don't like pleather earpads.
The perception of soundstage is a very complex topic where the shape of your body and the way your brain works play an important role.
I wouldn't say K812 has more depth than HD800. To give you an idea of how I perceive them, HD800 might have 15% more depth and 40% more width.
This is recording dependent (for sure) so take those numbers for what they are.
K812 has sharper imaging.
Both work well with quality chamber music and both can get the job done with relatively good quality videogame OST.
Keep in mind videogame OST is (more often than not) just ok recording quality (compared with top class audiophile labels)
The Beyerdynamic T1 is a nice pick for chamber and videogame OST, but it should be noted that based on many reviews and some measurements, it's not that easy to predict what kind of T1 you'll get.
I have a warm sounding and not overly bright late T1 1st Generation that would work very well for the listed genres, but some other people have leaner and significantly brighter T1s.
Now there's also a 2nd Generation that's supposed to be even warmer and darker, but you'll find quite different opinions on how they actually sound, some people say they are still very bright, some other people say they lack the air and refinement of a good T1.1. So... yeah, despite owning a very enjoyable T1, it's a bit hard to recommend. I'll probably buy a 2nd gen T1 sometime down the road to measure and compare with my 1st gen.
My T1 is more punchy in the mid bass than both HD800 and K812. It's also slightly softer in the treble and less prone to emphasize sibilance. It's more forgiving of poorly recorded music, more compact and more organic sounding (opposed to the more ethereal kind of sound of HD800 and K812). It extremely good in terms of micro-detail/texture of the sound, it's a very vibrant headphone indeed. Just a tad behind K812 and HD800 in terms of macro-detail, so not that great as a tool.