I'm by no means an expert in IEMs and their cables. I'm more of a headphone guy. That said, I do have the N40s and I have replaced the cables on them. I noted earlier in this thread a particular design flaw of the N40s that makes replacing cables mildly difficult. The port is quite close to the body so typical wrap-around ear replacement cables don't always work. I bought a FiiO replacement cable and filed down the edge to fit tightly in there and that worked (cost about $50). I also used this cable (
https://www.amazon.com/gp/product/B07ZQCH9CN/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&th=1) and it works well with the N40. Its not the best cable on the market but it fits the N40s without having to file anything down.
I definitely recommending getting a replacement cable. The stock cable with the N40s sucks, I replaced it the second I saw it. I don't think, however, you need to spend a ton of money on it. Something around $30 to $50 would be just fine. It also depends on what you're running these IEMs off of. Buying a $70 cable when you're using the iPhone dongle would be silly, so keep that in mind.