Anything “classical.” Or really, anything recorded live. I was listening to a live recording of Max Richter’s On The Nature of Daylight - already a beautiful piece in and of itself, but the recording had an enormous bass rumble that echoed throughout the venue.

Acoustic stuff like bluegrass, folk, rando singer with a guitar or piano, etc. all sound spectacular. Post rock stuff, like Sigur Ros, God Is An Astronaut, Explosions In The Sky, are perfectly suited as well. I’d argue that even stuff unsuited for the HD800S will still sound great. I listen to a lot of metal, punk, rock, etc. on them as well, and the sense of space/separation they give to those tracks always puts a smile on my face.

There’s a lot of semi-obscure American, European, and Japanese brands in the highest echelons. Audio-Note, FM Acoustics, VAC, and a lot more. The brands you named, while I’d consider high-end, are unfortunately only “popular” high-end in the eyes of audiophiles who are deep into the hobby (I don’t agree with this idea!). They certainly make good products, so don’t fret.

Bandcamp or equivalent sites. HDTracks is an option, but their selection is uninteresting.

I don’t think they exist. HDMI extractors exist, even ones with earc, but there aren’t any HDMI to analog cables. HDMI cables use a digital-only signal, and they need a decoder and a DAC to work. I don’t think there’s anything that can fit into a cable. And USB 2.0 already has all the bandwidth that a multichannel audio signal could ever need. Uncompressed 32bit/384khz x 2ch comes out to 24.576 mbps. You would need 20 channels of that to max out a USB 2.0 cable (480mbps max). 32/384 is already rare as fuck, and 20 or more channels of audio only exist in studios and professional environments. This is headphones, so all you need is 2 channels. On top of that, studios generally work with 24/96 at max, so that comes out to 4.6 mbps for 2ch.

I really suggest you do some more reading. 48gbps is meant for video, not audio.

You absolutely do not want to be blasting your ears with anything close to 144db for any amount of time, if you can help it. That gets into immediate hearing loss territory. Even 96db is hearing loss territory, though not immediate.

I usually listen at 75-77db, which allows for peaks of higher than 77db without problems. Even having 144db is kinda useless for me. The question of whether your gear can produce 144db of audio is in question as well. I’ve also read that microphones can’t really record anything above 20 bits - 120db, in other words.

I would argue that they’re pointless for any gear.