The F150c is kind of like a discovery earbud for me. It's on the cheap side but the sound is classy. I bought it after reading here in HF like years ago and just did not spend enough time to listen to it because I was so engrossed with other hyped earphones that I also bought to try like the Zen 1 & 2, Liebesleid and Turandot. Finally when I got tired of those earphones, I tried the F150c thinking I can just do casual listening in the office then throw it in my bag. However, a revelation happened to me and for the first time, I felt happy enjoying my music. It actually put a smile on my face. I'm not good with words specially the technical stuff but it is just natural and open sounding. It has enough bass, highs and mids and soundstage and separation for the music that I listen to. Maybe it's the tonality and how comforting it sounds like speakers instead of like earphones. In short, I like the sound signature. If I can wish for it to improve more is for the vocals to come out just a little upfront. Not too forward because I hate shouty sounds but maybe 10% more present. Even without it, I am a happy camper.
Take note though that I am listening to it via a USB DAC dongle which for me greatly affected the overall sound. There is a very comprehensive review of USB audio dongles by Mr. Andy that you can find here:
https://andyaudiovault.com/donglemadness/. He used to have a dedicated thread here at HF but something controversial happened that made him removed all his posts on that thread. He is a good guy and he gave me a totally new perspective of how some of these inexpensive dongles can help define and refine the sound of our audio equipment. Just using a smartphone with the dongle is like using a mid tier dedicated DAP. Sorry for the very long reply.
For the K64, it's actually K64S. I think the standard K64 is OFC and the K64S that I bought is using 6U-PET plug. I don't know if it's better but that's what I chose.