I have a feeling that the X5 is going to be another success story in the Fiio's lineup.
Fiio has always improved tremendously when it introduces a successor to a previous device .eg. E12 and E17 were substantial improvements over their predecessors E11 and E7, respectively. They take note of anything and everything that wasn't right or needed improvement, and go about implementing it their next device.
Similarly, the X3 could very well have served as a stepping stone in the process of creating X5. Hence, we can all be assured that the X5 won't disappoint anyone (well, at least most of us).
After going through various amps in the <$400 range, I still use the E12 as one of my primary headphone amps! So, I have high expectations for the X5, and I'm sure I'll not be disappointed.