Here are criteria I would consider for gym IEMs:
- Good bass (Most of the songs you will be listening in the gym will have a good beat to it)
- Isolation (Block out gym music)
- Deep insert or at least a good fit (So it doesn't fall out)
- Microphonics or ability to go overear
- Rugged (can handle sweat on the cord / impact on the housings)
- Low price
Sound quality can come second imo. I don't think you will enjoying the SQ difference of pavarotti's voice when you are busy deadlifting 200+ pounds lol.
That being said I would ask those who have the fx40 whether or not the build is good enough for gym use. To me it looks a tad fragile having no strain relief at the housing. Either way it's only 20$. I am personally using the MEE M16 atm, it's 9.99$ on amazon currently.