You'll probably want something on the warm side, for sure. When I borrowed the Stellia and a Headamp GS-X Mini amp (which I'd describe as really great neutral amp - I'm a big fan of Headamp), it sounded good - very punchy and dynamic, but also definitely too bright and tipped-up on top. It would get fatiguing after a while. By comparison, the Hifiman Susvara was PERFECTLY balanced on that same amp (very close to the best headphone sound I've EVER heard), and The Meze Empyrean was woken up a bit but was still too dark and thick for my tastes (I just don't love the Empyrean).
When I tried Focal Utopia on a Naim Integrated amp's headphone jack, it was actually a beautifully voiced combination, despite an annoying bit of ground hum. And Naim clearly voiced its headphone driver slightly to the warm side. So a Naim amp could be a good choice for Stellia too.
And then ECP Audio is an under-the-radar "cult" favorite amp maker that has small batch runs of some incredibly well designed headphone amps. All their stuff sounds amazing for the money. And I know they've made variant of some models specifically tweaked for Focal headphones, denoted by an "F" suffix. I don't know what models they're currently making these days, but they're always worth a look.