I finished many hours of Level Matched A/B listening sessions with the Ibasso DX90 and the Fiio X3ii.
Bottom line the winner, for me, is:.............the X3ii.
They are both exceptional DAP's, but price/performance and at half the cost, the Fiio X3ii gets the nod here. The X3ii is smaller and has the scroll-wheel interface that works well, but with a terrible screen with small font that is impossible to see outside. The Ibasso wins out on the UI front with its touchscreen, but loses groung with its gapless hiccups.
Sound-wise it got to the point where I had them both on the same level and exact spot on a track and there was almost no difference in the sound between the 2. You could say when switching from A/B that it was a transition without any noticeable difference at all. Longer sessions gave me the feel of a tiny bit of "aireness" with the DX and and a little more mid-bass punch with the X3ii.
Funny thing I found out was the more that I listened to one device and read reviews the more the perceived differences I would make. Maybe it is just me, but I must have a really short Audio memory, because no matter how much I tried when I was doing it the old style way I would hear more of a difference than when utilizing a true A/B device.