I've just had my RMA accepted, so my X5 is going back too. I was willing to wait for firmware updates, and then I started to experience pops and clicks during USB DAC usage, and when playing certain tracks from µSD in DAP mode. I decided enough was enough.
It's actually a massive relief! I don't think I ever bought a buggier product.
Uggh! If it weren't for how easy it is to return items through Amazon, reading this is enough to scare me into cancelling my order.
I'm not suggesting that you don't have a bad unit.. but when reading your post the first thing that came to me was you don't know how to set up the player options for using the X5 as a USB DAC... its not easy to get all the settings correct for perfect playback...
I'm hoping that Fiio moves their USB Driver to ASIO and it might get easier and better sounding...
And therein lies the problem with every DAP, it seems - with the X5 being the least troublesome of them all. The user has to accommodate the DAP's failings - enabling it to function sufficiently to meet lowered expectations.
The wide variation in response to the X5 is partly due to the variations in how the product is deployed (some functions are buggier than others), but also due to extremes of willingness to adapt to disappointment.