I think Schiit Mjolnir 2 is worth considering, its slightly U-shaped signature,powerful bass and dynamics will do the headphone justice. It is a plug and play amp, no tube rolling nervosa and it is flexible enough to be used with the solid state 'tubes' from Schiit if you are curious. Don't forget a decent source as well!I read that the Auteur is relatively easy to drive - so can a modest-power tube amp do it well? If so, what's a good suggestion for something that won't break the bank? I planned on using a lake people RS02, which I can eq warmer, but should I look at something tube?
IMO Cavalli Liquid Carbon would be fine as well.
Try the Feliks Elise (non 2018 version), I think it may be a good match.
I briefly had the Cayin IHA-1. Very clean sounding amp with a slight treble tilt. A good pair if you find the Auteurs a tad dark.
The Auteur does not need alot of power to drive.
As always, try to demo gear before committing.