The only tubed headphone amp I've owned is the one in my preamp, so I won't make any recommendations there, other than to say I would give the Schiit Mjolnir 2 some consideration. I owned the first one and it did a lot of things right.
If you like the Harbeth sound, you might like Hifiman, Audeze, or Sony MDR-Z1R headphones. They are all close to neutral with nice detail, good mids, and maybe a little on the "warm" side.
If you like the Harbeth sound, you might like Hifiman, Audeze, or Sony MDR-Z1R headphones. They are all close to neutral with nice detail, good mids, and maybe a little on the "warm" side.