Well for those that doesn't want tube amps and don't want to be spending over a kidney, a lung and some vital organs, I find the solid state Aune X1 Pro to be the best fit with the Fidelio X1, period (unless you like a wildly different signature, like more bass).
After over a week of owning them, I can confidently say the X1 Pro does the Fidelio X1 justice. I've already written my early impressions a few pages back, but the TLDR is; tighter bass, more forwrard, fluid mids, even more energy in the treble without sounding harsh.
Power is there to boot and it improves the dynamics of the Fidelio X1 over something like say the popular E17 pairing.
How much is it? I paid $195 for it. Yes, $195. That's cheaper than a Schiit M&M stack and it comes with even more inputs for convenience sake.
The bonus? It has support for DSD files too.
Aune has limited 100 units for its first launch, but I'm pretty sure wtih the feedback so far, they are eager to ramp up production on the X1 Pro along with the new T1 MK2.