My experience has been the same. I can't differentiate between ak100ii to the AK240. I also know I'd have a hard time picking between my phone (when not using viper) and pretty much any dap I've owned.
The differences for me, come when I apply eq, or if on Android root, add viper, and use a different music player. Otherwise, everything is a blur.
I recently was loaned the LPG, damn is that a great dap. PMEQ, is one of the big things along with the ridiculous power of the LPG.
I also now on the Fiio X7. I love this dap. It's quite good. But, one small caveat, the amp module included is very weak. It doesn't drive all of my IEMS well. Add the am2, big difference.
I still have my Fiio X1 and X3 OG. Both very good players, but not as quiet, missing that black background. The X3 has an extremely fun signature and the hardware buttons for bass and treble are great. The X3 would regularly receive playing time, even when I had the AK240 in my stable.