It’s a long time since I’ve seen a review of a device like this! I used too have a cheap FiiO but it never really seemed to make much difference. I believe Helm also do a USB DAC/Headphone Amp dongle (Bolt?) which is also THX so that would be interesting to see reviewed.
I've no idea why you're running into that problem. It plays every track that's in my apple "music" formerly "itunes" library. No probs with ripped CDs or even certain audio pwned from youtube, it plays them all on my iphone, then through the ONKYO HF Player app.