Tube OTLs are good medicine for Sennheisers.
What do you mean the ADI-2 FS doesn't have USB input?
Some of the FS models appear to not have a USB input, while the non-FS and the Pro FS do.The RME ADI-2 DAC does possess one USB-B input connector.
Still, I kinda want to have an R2R dac rather than a DS one, not only for my HD800S, but also for my Stax SRM-353X energizer with L700 Mk II earspeakers.
The Denafrips Ares II has NOS and OS modes, with two filters for the OS mode. It's just a matter of finding the best mode for each setup.
The Bifrost doesn't have different modes, for that reason I would much rather go for its bigger brother, the Gungnir MB, just for the HD800S alone.