The IEM question is really one of several variables I think.
Starting with the IEM's themselves you need to ask yourself what type of sound profile you desire. I.E. flat and neutral or more scooped or bass hyped? Based on your answer to that question you will immediately narrow your field of options. I don't feel there is necessarily a wrong answer here. In theory, a more neutral sonic profile would give you a more accurate presentation of sound mix (including that of your bass). But, when in the moment playing live on stage, sometimes what matters more than accuracy is a sense of "feeling" your playing. That can lend itself to potentially selecting more of a bass hyped sonic presentation. Some have a wider sound stage which some like and some don't. Others will hype what is called the "separation" between frequencies (Lows, mids, and highs) with lower priced IEM's stereotypically suffering from little separation and blurring of frequencies. It is for this reason that even after using very expensive IEM's I now use the KZ ZS10 Pros due to their tendency to hype the bass a little but not over the top. They are a good balance of what I like with the benefit of not costing a lot.
Then you have the fit of the IEM's. Custom molded or not. Size and weight of the earpieces. How well they seal. And IMHO not as important, but how they look.
Then you have the question of the source audio. The mix being fed to you ultimately determines what you hear and whether you like what you hear or not. In some scenarios, musicians don't have control of each individual "signal" in the mix. I.E. drums, guitar, vocals, keys, etc. In other scenarios, musicians have their own Aviom or Livemix interface allowing for independent control of each channel (or signal). Going even another step further, sometimes musicians have access to independent channels for multiple signals from the same instrument. I.E. separate channels for each of drum toms, kick drum, snare drum, etc. Or a channel for low keys and a channel for high keys. Or maybe a live presence microphone(s). In other words, to what degree do you have control? The Aviom or Livemix also allows for EQ modification and things like reverb and stereo or balanced mix. At the end of the day, the source audio (and what you include, or more importantly, exclude) is going to have a big impact on how you interpret what you are hearing. I've been in scenarios where what was fed to me sounded like crap and it didn't matter what IEM's I was using, I wasn't going to be happy.
Finally, the sensitivity of your ears. How do you hear frequencies? What is important to you in what you want to hear? This sort of brings us back to the top — different IEM's cater to different sonic stages.