I ran my comparison between the dx260 and the M15s again after 200+ hours of burn in on the dx260. Allowing for the vagaries of amateur A/B-ing between two very similar DAPs, my conclusions were, I think, interesting. Reflecting back on my earlier experience at 75 hours, the sibilance on the 260 is tamed, the mids have settled back, losing any sense of jangle or over-brightness. As such, after an hour and a half of back and forth between the two, I would say the dx260 slightly edges it. The Ibasso has a touch more resolution throughout, the base is tighter and more realistic; the timbre is still bright, but this just adds to the detail rather than being sparkly - it makes the Fiio sound a bit dull by comparison. That said, the M15s does have one advantage SQ-wise: it's more resolving and that makes it more forgiving when the music gets very complex (e.g., big symphonic pieces) or isn't brilliantly well recorded (e.g., some vintage material). Further burn in on the 260 may even the score here. Obviously, the M15s has way more power, especially in desktop mode, and has a lot more funky options (e.g., all to DSD - which I didn't use). One disadvantage for the iBasso is its very sensitive buttons and volume wheel, which make putting into or taking it out of a pocket very likely to shift the track. The M15s has a switch on the side which locks the buttons and the touch screen.
There is very little in it, and could easily see someone taking a very different view. But it would be down to taste (and pocket size - the FiiO is big!) rather than technical competency.
Both DAPs were set to 'slow roll off'; the Fiio had the 'second harmonic regulation' set to level 1. The FIR on the dx260 was off.
Edit - after more sustained listening than simple a/b-ing, I began to find the brightness of the dx260 fatiguing and the lack of resolution disengaging and have happily gone back to the M15s - while the dx260 has gone back to the shop.
