Sorry,
I have never heard opus so I have no idea how it sounds
X5-3 is neutral but smooth. The top end can be listened well at loud volumes. X5-2 has a more neutral sound, and it's specific signature works well for low volumes.
Both signatures can be reasonably equalized and x5-3 is a really interesting dap in operation, but I also love x5-2 .
The only dap from another company I had a chance to test in true depth was 1Z which had way way too little volume (shockingly so ) and it's sound was far too warm and rolled off.
From Fiio, I generally think that all daps are extraordinary at their price points, x5ii being the more neutral variant, x5-3 being the smooth variant.
X7+am2 is smooth while x7+am3 is energetic and neutral.
At any rate I like all of them and I think that all of them are great devices , especially at their price points, you should test and pick which sounds best to you and your tastes. Fiio had an offering for anyone as far as I've tested.
And I'm not affiliated, but avstore has mostly fiio daps in their showroom so I got a chance to play with them in depth. Sadly, no Sony daps didn't really impress me as much and I prefer to always comment on what I like and what sounds good , I'm not a complainer by my own personality. That is because I'm grateful for living in Bucharest, grateful for being able to hear all of those and grateful for discovering this hobby at a young age. I'm sure there is a public for any dap, but I recommend fiio out of experience, buying their products and receiving nice , fast, and precise support. Got an issue solved in 15 minutes. I had to file a report with Romanian customer protection for getting both more expensive and cheaper products , fiio is exemplary and I respect this .
As an answer to your original question, x5ii is not smooth , so it might be more neutral.
You can also wait for x3-3 and x7proii, they might be neutral, I don't have any idea , but I'm curious to hear them as well!