Flares Pro just sounds like that. You don't have a faulty unit. I've had two Flares Pro (and send one for check-up). Some people (many actually) just don't pay attention to some things in sound and don't listen closely enough.
First of all - I've tried many different sources and the truth is nothing sounded as good as their bluetooth module. Of course, bass was tightier and more impactful on some of them, etc. but as a whole the bluetooth module is best I've heard.
Second, choosing the right tips is extremely important. Comply's are best to eliminate your problem but at the same time, they take away a bit of clarity, texture and make soundstage smaller... INAirs are a quite good compromise - somewhat smoothing the harshness (not completely though), without taking anything away that much.
Same goes with Flares Gold. Unfortunately "the tizz/fizz" is still there - it's the same driver after all but it's better to some degree. Fortunately, with Comply's it's completely gone and this time without the downsides like with Flares Pro.
I don't have many sources right now to test Flares Gold wired but with Shozy Alien+ (class A amp section) and Sabaj DA3 (cheap but great DAC/amp - I like it better than Mojo or Oppo HA-2) it sounds worse than with bluetooth module. Same goes with Sony WM1A or Galaxy S8+.
For long sessions I still prefer R2pro's. I think they're the best IEMs from Flare and my favourite IEMs on the market (sadly no longer on the market). Not as good technically as PROs and GOLDs but still great and much more musical and natural sounding.