Tubes are a lifestyle decision. Gotta love 'em, but if you go down that path, forever will it control your destiny...
I think that there is no such thing as a totally "straight wire with gain." We wish there were, but every amp interacts with every headphone differently due to the source impedance, the load impedance, the cable, and the unique signature of the amp. Solid state can vary as much as tubes, but unless it's rolling op amps, it is impossible to change if you're not happy.
Plus every headphone reacts with every ear/brain interface differently. So there is some combination that will generally work best for you and give you either what you perceive as "most neutral" or give you a specific color you find most natural or pleasing, and that may or not feel "neutral" to you.
I use tube and SS gear. It's kind of a mood thing, I've never found that any one piece of gear is always best, and sometimes it's music genre dependent, which is why I now have seven amps, I guess.
Welcome to headphone hell. I know there is a desk under the equipment somewhere...