The treble of R6 is much better than that of X5-3
By better, I mean much more neutral, natural and well expressed. In this aspect, X5-3 always came to me as a DAP that is too smooth, and I mentioned this many times in my review, even as a negative.
R6 is only slightly warm and slightly thick, without being more than a touch smooth, so you can expect a signature that is pretty neutral, at least with earphohnes and headphones that are of higher impedance. Since results may be those of a brighter sound if you're using a IEM or Headphone that has a really low impedance, I can't say if it doesn't become even more neutral, (given its high output impedance), but besides hiss, I haven't really noticed any kind of downside to that high output impedance, and I personally can't hear hiss while playing music anyways
If you're planning on getting a new DAP, the best choices from what I tried and reviewed so far are X7mkii, DX200, R6 and Opus #2, but each for a different reason