Visiting an acquaintance who is in one of my fantasy football leagues^^^ He owns the K9 Pro (ESS), so got the chance to try it a bit this morning. *Disclaimer: I moved away from desktop rigs a decade+ ago, so I'm just kinda spit-balling here*
Seems to be a pretty convenient AIO solution; solidly built. He's a Mac user like me and currently just connects directly to his M1Pro Macbook Pro via USB (he's got a green colored Audioquest USB-C to USB-B cable); says he's considering a USB interface to have between them (??). Back in the day, I think we just called them SPDIF converters...anyway, not sure why he feels he needs that. I noticed zero issues going direct between the two units via USB.
Sound wise, nothing really stood out. Seems like a competent piece of gear.
At the SRP of $850USD, this really should have a remote control included, IMO. In fact, and again, considering my disclaimer above, this feels a bit overpriced to me, but as always, YMMV.
I heard one of those Topping A90/D90 (?) stacks recently and found while it had gobs of power, sound came across as sterile to me.
The K9, to me, doesn't come across that way, so the winner in that regard in comparison at only ~60% the cost of the Topping stack.
I have no need for such a device, but if I ever do once again delve into desktop rigs, I'll at least give FiiO a look to see how things have progressed for them; see if they learn lessons in development (not all companies do, sadly).