IMHO, sound is improved from the AP80 Pro. This is just from memory, as I sold my AP80 Pro months ago, but to me the sound is more full-bodied and balanced. A little more bass. Detail retrieval is really pretty outstanding for the price. I bought this thing on a whim, but I'm glad I did. The 2.5mm port is ok with me because I have been using a Shanling M2X, which also has 2.5mm. Plus I use ddhifi adapters when necessary. Trying to decide now whether I actually prefer the AP80 Pro-X to my M2X, which I have been really loving. So far I have been mostly listening to it through my ATH-MSR7b and it's a really great pairing.
Firmware appears to be the same, but I have not noticed any bugs yet. Very VERY slight wobble on the volume wheel. Tbh I had not even noticed yet until I checked it out just now.