ADI 2/4 Pro SE has crazy amount of functions, which is most likely the reason it is so expensive, but based on my impressions, in the music playback alone Fiio matches RME. They sound extremely similar to my ears. I could possibly tell them apart by listening to specific songs, looking for those subtle differences (e. g. in the song Spell by Uriah Heep, Ken Hensley voice in the 3rd and 6th verse comes across much harsher on HE1000se with RME than with Fiio - it's a tiny difference, but Fiio is a bit smoother), but if I was blindfolded and you played a random song, I would be just guessing. In my opinion Fiio is a great alternative to RME - if you don't need additional functions, you can have pretty much the same sound while saving roughly 1000 euros.
It is a bit annoying with screen - it will eventually dim down (30 min. is the longest it can be set) and when it does, you can change the volume with the knob or the remote, but the screen won't light up, so you can never be sure what the volume is, unless the screen lights up. To light up the screen you need to click the volume knob or the specific button on the remote, which is can be annoying if you're someone who changes volume a lot - bear in mind that when it lights up, the screen will stay that way for 30 min. if you set it this way, so I wouldn't say it's terrible, just an inconvenience. I think RME has a better way of dimming screen - it's either on all the time or off all the time (after couple of seconds of inactivity), however, if it dims down it will immediately lights up if you do something, like change the volume for example - that way you always have confirmation of what you did and after a couple of second screen dims down again (if you set it this way of course).
The occasional lag is also not a deal breaker. I noticed it when connected to PC via usb. It usually happens when you first start the device and when the is no signal going in for a longer period of time. For example I was working from home on Friday, but a had to really focus on task for about roughly 40 min. so I turned off the music to keep the distraction out. After I it was done and I wanted to resume playback and there was about a half a second delay between pressing a button and hearing music. Occasionally I also hear a soft click with the delay. It's not a big deal for me, music doesn't lag, it works well for movies and gaming as well. I just thought I mention this as I have never experienced something like this any RME device. I never had any issues with any RME device. My old ADI-2 DAC I must have used for thousands of hours over the years, no lags, no delays, nothing. It was like a tank, ever-reliable.
The reason why I'm considering swapping to Fiio is convenience. It runs cooler, has 4pin XLR headphone output and it has more power available, if I would need it. I will test the EQ function next week and if it works well enough, I think I will replace ADI 2/4 Pro SE with it as my daily driver.
Soundstage is medium sized when it comes to width and height - meaning that you hear sounds just outside your head. With HE1000se it's a fair distance outside, with Expanse it was just outside. Depth is rather shallow, it's not a wall of sound, but far from holographic. It's pretty much the same as with ADI 2/4 Pro SE (bear in mind that ADI 2/4 Pro SE has a wider and slightly taller soundstage with balanced output than ADI-2 DAC and even ADI-2 Pro).
Imaging and separation is pretty good, I'd say one of its strengths - it is quite easily to identify individual sounds and position them in space, even with very busy parts of the songs. Again though, it's on par with ADI 2/4 Pro SE, they are both very good in this area.