I own the MCTH and SDAC unit, and i would like to highlight a few potential issues with sources. Many people do not realise the importance of the quality of the source. If I hook up the SDAC to my laptop with the supplied micro usb cable it souds like crap. This is because of interference on the motherboard of the laptop causing jitter and noise in the power supplied to the SDAC. The SDAC takes its power and digital information from the source over the cable. If I switch to the HiFimeDIY Sabre 9018 and connect that to the MCTH I get much improved results. This is because of its superior power filtering and jitter rejection. If I play music from my LG G6 the quality improves again but not as much.
Units such as your Fiio are specifically designed with electrical isolation and quality of digital feed in mind. You will probably be better off using that, even if your digital source doesnt suffer from noise or jitter, which, if its a Windows laptop, is unlikely.