What do you think of the Bravo Audio V2, which is a tube amp? Sells at about 55€ in Germany.
I've heard some people prefer tube amps and I think I read about the DT990 to work especially well with tubes.
Edit: Just read they aren't good quality.
The Bravo is what I'd like to think of as an "introduction to tubes" amp. The "Indeed" brand is essentially the same amp with slightly better parts, I'd recommend that one. I say "intro" amp because you'll probably be quite pleased at first, especially if you swap (roll) different tubes and play around that way, but eventually the crazy volume gain and cheapo build quality will annoy you, and the amp will probably fail within a year or two.
I personally skipped to a bit more expensive amp that was better built and including better parts, bought a few New Old Stock tubes. It is my favorite amp, really seems to help mids become magical while also increasing the perception of soundstage size and distinctness, which obviously is a benefit with Headphone Surround gaming. I've debated on lending it to Mad a few times, but I don't want to burden him with surprise responsibility, and he wasn't particularly happy with his last Tube amp experience.