Welcome to the forum!
I think something @DarthPool said kinda resonates with my thought between the two… the Phonitor kinda gets out of the way, where I think the FA-10 drives. I don’t think either colors anything and I may argue that the Flux is cleaner and, while the stage on the Phonitor is good, the Flux bests it. Each lend themselves well to different types of headphones and preference. The Phonitor has more features and a far better pot on it (it’s probably my favorite of all the pots so far). Neither are going to give that low end that you may find in some other amps (iFi, Schiit, etc).
For planars (well, Audeze ones I’ve tried it with so far at least), I’d take the FA-10 hands down over the Phonitor. They love that class-A power and I don’t think I’ve heard my LCD-X sound better. It should be noted that I’m a sucker for soundstage and the FA-10 has it better than most amps I’ve heard.
I haven’t heard the FA-12 yet but maybe soon